ng_pktbuf: simplify API by extension
ng_pktbuf: simplify API by extension

This simplifies the `ng_pktbuf` API by adding a new function
`ng_pktbuf_mark()` which takes over some functionality of
`ng_pktbuf_add()`. `size == 0` for `ng_pktbuf_add()` is now illegal.

MohmadAyman/RIOT,adrianghc/RIOT,rousselk/RIOT,openkosmosorg/RIOT,robixnai/RIOT,gautric/RIOT,dailab/RIOT,khhhh/RIOT,Yonezawa-T2/RIOT,bartfaizoltan/RIOT,MohmadAyman/RIOT,shady33/RIOT,neiljay/RIOT,JensErdmann/RIOT,mtausig/RIOT,dailab/RIOT,neiljay/RIOT,jasonatran/RIOT,altairpearl/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,immesys/RiSyn,aeneby/RIOT,avmelnikoff/RIOT,watr-li/RIOT,LudwigOrtmann/RIOT,FrancescoErmini/RIOT,bartfaizoltan/RIOT,ntrtrung/RIOT,DipSwitch/RIOT,automote/RIOT,MarkXYang/RIOT,shady33/RIOT,lazytech-org/RIOT,mfrey/RIOT,plushvoxel/RIOT,A-Paul/RIOT,binarylemon/RIOT,automote/RIOT,bartfaizoltan/RIOT,kb2ma/RIOT,TobiasFredersdorf/RIOT,MarkXYang/RIOT,A-Paul/RIOT,msolters/RIOT,dkm/RIOT,JensErdmann/RIOT,adjih/RIOT,dkm/RIOT,biboc/RIOT,beurdouche/RIOT,syin2/RIOT,arvindpdmn/RIOT,FrancescoErmini/RIOT,openkosmosorg/RIOT,abkam07/RIOT,jremmert-phytec-iot/RIOT,basilfx/RIOT,l3nko/RIOT,jasonatran/RIOT,RBartz/RIOT,dhruvvyas90/RIOT,gebart/RIOT,avmelnikoff/RIOT,kb2ma/RIOT,JensErdmann/RIOT,ks156/RIOT,BytesGalore/RIOT,attdona/RIOT,hamilton-mote/RIOT-OS,altairpearl/RIOT,Yonezawa-T2/RIOT,daniel-k/RIOT,gbarnett/RIOT,gbarnett/RIOT,patkan/RIOT,kaspar030/RIOT,RubikonAlpha/RIOT,beurdouche/RIOT,lebrush/RIOT,cladmi/RIOT,MonsterCode8000/RIOT,kaspar030/RIOT,neumodisch/RIOT,jremmert-phytec-iot/RIOT,EmuxEvans/RIOT,Yonezawa-T2/RIOT,altairpearl/RIOT,plushvoxel/RIOT,adjih/RIOT,wentaoshang/RIOT,altairpearl/RIOT,thiagohd/RIOT,smlng/RIOT,alignan/RIOT,arvindpdmn/RIOT,ant9000/RIOT,syin2/RIOT,bartfaizoltan/RIOT,Ell-i/RIOT,abp719/RIOT,haoyangyu/RIOT,RIOT-OS/RIOT,rousselk/RIOT,roberthartung/RIOT,jasonatran/RIOT,kerneltask/RIOT,malosek/RIOT,mziegert/RIOT,plushvoxel/RIOT,thiagohd/RIOT,authmillenon/RIOT,alignan/RIOT,kb2ma/RIOT,l3nko/RIOT,kaleb-himes/RIOT,gbarnett/RIOT,roberthartung/RIOT,kaspar030/RIOT,daniel-k/RIOT,kbumsik/RIOT,zhuoshuguo/RIOT,asanka-code/RIOT,kb2ma/RIOT,malosek/RIOT,OlegHahm/RIOT,robixnai/RIOT,watr-li/RIOT,asanka-code/RIOT,ks156/RIOT,MarkXYang/RIOT,kbumsik/RIOT,Darredevil/RIOT,ntrtrung/RIOT,neiljay/RIOT,brettswann/RIOT,hamilton-mote/RIOT-OS,BytesGalore/RIOT,thiagohd/RIOT,openkosmosorg/RIOT,alignan/RIOT,Hyungsin/RIOT-OS,rfuentess/RIOT,jbeyerstedt/RIOT-OTA-update,rakendrathapa/RIOT,josephnoir/RIOT,asanka-code/RIOT,katezilla/RIOT,OlegHahm/RIOT,TobiasFredersdorf/RIOT,kYc0o/RIOT,attdona/RIOT,aeneby/RIOT,mziegert/RIOT,robixnai/RIOT,LudwigOrtmann/RIOT,A-Paul/RIOT,beurdouche/RIOT,BytesGalore/RIOT,immesys/RiSyn,gautric/RIOT,miri64/RIOT,toonst/RIOT,RubikonAlpha/RIOT,authmillenon/RIOT,shady33/RIOT,zhuoshuguo/RIOT,rfuentess/RIOT,syin2/RIOT,backenklee/RIOT,jbeyerstedt/RIOT-OTA-update,thiagohd/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,shady33/RIOT,DipSwitch/RIOT,RIOT-OS/RIOT,dhruvvyas90/RIOT,dailab/RIOT,d00616/RIOT,RBartz/RIOT,stevenj/RIOT,jasonatran/RIOT,daniel-k/RIOT,RBartz/RIOT,brettswann/RIOT,gebart/RIOT,OTAkeys/RIOT,plushvoxel/RIOT,ks156/RIOT,thomaseichinger/RIOT,x3ro/RIOT,d00616/RIOT,avmelnikoff/RIOT,RubikonAlpha/RIOT,mtausig/RIOT,wentaoshang/RIOT,luciotorre/RIOT,dkm/RIOT,brettswann/RIOT,alex1818/RIOT,LudwigOrtmann/RIOT,robixnai/RIOT,yogo1212/RIOT,malosek/RIOT,tfar/RIOT,jfischer-phytec-iot/RIOT,mtausig/RIOT,dkm/RIOT,ntrtrung/RIOT,rakendrathapa/RIOT,automote/RIOT,arvindpdmn/RIOT,thomaseichinger/RIOT,ThanhVic/RIOT,jbeyerstedt/RIOT-OTA-update,ant9000/RIOT,Hyungsin/RIOT-OS,basilfx/RIOT,ks156/RIOT,brettswann/RIOT,haoyangyu/RIOT,openkosmosorg/RIOT,smlng/RIOT,LudwigKnuepfer/RIOT,DipSwitch/RIOT,yogo1212/RIOT,Osblouf/RIOT,thiagohd/RIOT,brettswann/RIOT,kaleb-himes/RIOT,LudwigKnuepfer/RIOT,kYc0o/RIOT,zhuoshuguo/RIOT,LudwigKnuepfer/RIOT,backenklee/RIOT,patkan/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,x3ro/RIOT,FrancescoErmini/RIOT,biboc/RIOT,Josar/RIOT,miri64/RIOT,Yonezawa-T2/RIOT,EmuxEvans/RIOT,msolters/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,ant9000/RIOT,latsku/RIOT,alex1818/RIOT,biboc/RIOT,MohmadAyman/RIOT,abp719/RIOT,Lexandro92/RIOT-CoAP,herrfz/RIOT,mtausig/RIOT,watr-li/RIOT,immesys/RiSyn,abkam07/RIOT,neiljay/RIOT,haoyangyu/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,asanka-code/RIOT,mfrey/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,mfrey/RIOT,kbumsik/RIOT,daniel-k/RIOT,Ell-i/RIOT,rakendrathapa/RIOT,lazytech-org/RIOT,kaspar030/RIOT,ant9000/RIOT,latsku/RIOT,BytesGalore/RIOT,binarylemon/RIOT,kerneltask/RIOT,rakendrathapa/RIOT,authmillenon/RIOT,kbumsik/RIOT,ant9000/RIOT,Hyungsin/RIOT-OS,kYc0o/RIOT,alignan/RIOT,jbeyerstedt/RIOT-OTA-update,kaspar030/RIOT,JensErdmann/RIOT,x3ro/RIOT,alex1818/RIOT,d00616/RIOT,adrianghc/RIOT,neumodisch/RIOT,katezilla/RIOT,binarylemon/RIOT,thiagohd/RIOT,lazytech-org/RIOT,cladmi/RIOT,altairpearl/RIOT,rajma996/RIOT,Darredevil/RIOT,miri64/RIOT,attdona/RIOT,herrfz/RIOT,haoyangyu/RIOT,thomaseichinger/RIOT,dhruvvyas90/RIOT,wentaoshang/RIOT,ThanhVic/RIOT,ntrtrung/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,abkam07/RIOT,neiljay/RIOT,FrancescoErmini/RIOT,Lexandro92/RIOT-CoAP,d00616/RIOT,asanka-code/RIOT,lazytech-org/RIOT,robixnai/RIOT,malosek/RIOT,gebart/RIOT,patkan/RIOT,alex1818/RIOT,EmuxEvans/RIOT,dailab/RIOT,beurdouche/RIOT,dhruvvyas90/RIOT,binarylemon/RIOT,latsku/RIOT,khhhh/RIOT,stevenj/RIOT,rakendrathapa/RIOT,mtausig/RIOT,hamilton-mote/RIOT-OS,openkosmosorg/RIOT,Osblouf/RIOT,authmillenon/RIOT,Darredevil/RIOT,beurdouche/RIOT,biboc/RIOT,TobiasFredersdorf/RIOT,toonst/RIOT,FrancescoErmini/RIOT,l3nko/RIOT,RBartz/RIOT,BytesGalore/RIOT,luciotorre/RIOT,Hyungsin/RIOT-OS,stevenj/RIOT,katezilla/RIOT,gautric/RIOT,abp719/RIOT,ntrtrung/RIOT,Lexandro92/RIOT-CoAP,kaleb-himes/RIOT,bartfaizoltan/RIOT,msolters/RIOT,jfischer-phytec-iot/RIOT,binarylemon/RIOT,rfuentess/RIOT,robixnai/RIOT,luciotorre/RIOT,tfar/RIOT,ntrtrung/RIOT,RIOT-OS/RIOT,watr-li/RIOT,yogo1212/RIOT,khhhh/RIOT,rajma996/RIOT,MonsterCode8000/RIOT,dailab/RIOT,Ell-i/RIOT,tfar/RIOT,patkan/RIOT,ThanhVic/RIOT,gebart/RIOT,msolters/RIOT,abp719/RIOT,yogo1212/RIOT,gbarnett/RIOT,josephnoir/RIOT,avmelnikoff/RIOT,daniel-k/RIOT,backenklee/RIOT,MohmadAyman/RIOT,Darredevil/RIOT,bartfaizoltan/RIOT,LudwigKnuepfer/RIOT,binarylemon/RIOT,jfischer-phytec-iot/RIOT,lebrush/RIOT,lazytech-org/RIOT,zhuoshuguo/RIOT,luciotorre/RIOT,Josar/RIOT,l3nko/RIOT,MarkXYang/RIOT,neumodisch/RIOT,aeneby/RIOT,Yonezawa-T2/RIOT,alex1818/RIOT,katezilla/RIOT,altairpearl/RIOT,hamilton-mote/RIOT-OS,OTAkeys/RIOT,mziegert/RIOT,EmuxEvans/RIOT,josephnoir/RIOT,gautric/RIOT,x3ro/RIOT,Hyungsin/RIOT-OS,jbeyerstedt/RIOT-OTA-update,rousselk/RIOT,rousselk/RIOT,DipSwitch/RIOT,JensErdmann/RIOT,backenklee/RIOT,alignan/RIOT,kerneltask/RIOT,rousselk/RIOT,MarkXYang/RIOT,jfischer-phytec-iot/RIOT,stevenj/RIOT,aeneby/RIOT,watr-li/RIOT,haoyangyu/RIOT,gbarnett/RIOT,khhhh/RIOT,MonsterCode8000/RIOT,miri64/RIOT,katezilla/RIOT,authmillenon/RIOT,mfrey/RIOT,jremmert-phytec-iot/RIOT,rajma996/RIOT,wentaoshang/RIOT,attdona/RIOT,rfuentess/RIOT,A-Paul/RIOT,latsku/RIOT,mziegert/RIOT,automote/RIOT,tfar/RIOT,adrianghc/RIOT,EmuxEvans/RIOT,haoyangyu/RIOT,lebrush/RIOT,automote/RIOT,basilfx/RIOT,daniel-k/RIOT,basilfx/RIOT,abkam07/RIOT,yogo1212/RIOT,backenklee/RIOT,l3nko/RIOT,OlegHahm/RIOT,Yonezawa-T2/RIOT,Lexandro92/RIOT-CoAP,RubikonAlpha/RIOT,rajma996/RIOT,d00616/RIOT,stevenj/RIOT,stevenj/RIOT,immesys/RiSyn,immesys/RiSyn,shady33/RIOT,jremmert-phytec-iot/RIOT,herrfz/RIOT,luciotorre/RIOT,khhhh/RIOT,adrianghc/RIOT,cladmi/RIOT,MarkXYang/RIOT,zhuoshuguo/RIOT,kYc0o/RIOT,attdona/RIOT,dhruvvyas90/RIOT,MonsterCode8000/RIOT,adjih/RIOT,x3ro/RIOT,abp719/RIOT,OTAkeys/RIOT,hamilton-mote/RIOT-OS,abkam07/RIOT,patkan/RIOT,openkosmosorg/RIOT,rajma996/RIOT,tfar/RIOT,OlegHahm/RIOT,jasonatran/RIOT,ThanhVic/RIOT,ThanhVic/RIOT,LudwigKnuepfer/RIOT,kbumsik/RIOT,patkan/RIOT,adjih/RIOT,basilfx/RIOT,MohmadAyman/RIOT,authmillenon/RIOT,adrianghc/RIOT,RubikonAlpha/RIOT,watr-li/RIOT,Josar/RIOT,dhruvvyas90/RIOT,Lexandro92/RIOT-CoAP,roberthartung/RIOT,josephnoir/RIOT,roberthartung/RIOT,DipSwitch/RIOT,shady33/RIOT,mfrey/RIOT,kerneltask/RIOT,LudwigOrtmann/RIOT,Lexandro92/RIOT-CoAP,d00616/RIOT,rajma996/RIOT,automote/RIOT,smlng/RIOT,Ell-i/RIOT,attdona/RIOT,toonst/RIOT,MohmadAyman/RIOT,herrfz/RIOT,gebart/RIOT,lebrush/RIOT,neumodisch/RIOT,TobiasFredersdorf/RIOT,gbarnett/RIOT,adjih/RIOT,Darredevil/RIOT,Osblouf/RIOT,EmuxEvans/RIOT,dkm/RIOT,RIOT-OS/RIOT,malosek/RIOT,roberthartung/RIOT,rousselk/RIOT,kerneltask/RIOT,khhhh/RIOT,Josar/RIOT,alex1818/RIOT,thomaseichinger/RIOT,syin2/RIOT,arvindpdmn/RIOT,l3nko/RIOT,rfuentess/RIOT,kaleb-himes/RIOT,ThanhVic/RIOT,RubikonAlpha/RIOT,wentaoshang/RIOT,zhuoshuguo/RIOT,msolters/RIOT,gautric/RIOT,kaleb-himes/RIOT,biboc/RIOT,josephnoir/RIOT,A-Paul/RIOT,jfischer-phytec-iot/RIOT,aeneby/RIOT,RBartz/RIOT,Osblouf/RIOT,arvindpdmn/RIOT,neumodisch/RIOT,neumodisch/RIOT,asanka-code/RIOT,TobiasFredersdorf/RIOT,kb2ma/RIOT,toonst/RIOT,lebrush/RIOT,herrfz/RIOT,luciotorre/RIOT,brettswann/RIOT,MonsterCode8000/RIOT,DipSwitch/RIOT,immesys/RiSyn,ks156/RIOT,Josar/RIOT,abp719/RIOT,jremmert-phytec-iot/RIOT,yogo1212/RIOT,cladmi/RIOT,smlng/RIOT,miri64/RIOT,Osblouf/RIOT,msolters/RIOT,lebrush/RIOT,mziegert/RIOT,toonst/RIOT,jremmert-phytec-iot/RIOT,cladmi/RIOT,Osblouf/RIOT,FrancescoErmini/RIOT,latsku/RIOT,avmelnikoff/RIOT,MonsterCode8000/RIOT,abkam07/RIOT,wentaoshang/RIOT,LudwigOrtmann/RIOT,plushvoxel/RIOT,smlng/RIOT,RBartz/RIOT,Darredevil/RIOT,JensErdmann/RIOT,herrfz/RIOT,malosek/RIOT,arvindpdmn/RIOT,thomaseichinger/RIOT,LudwigOrtmann/RIOT,Ell-i/RIOT,syin2/RIOT,rakendrathapa/RIOT,mziegert/RIOT,latsku/RIOT,kYc0o/RIOT