gnrc_pkt: introduce packet list operations
gnrc_pkt: introduce packet list operations

Up until now `utlist.h` is used to manipulate the packet list within a
GNRC pkt snip. Since I always wanted to see if there would be an
advantage when using `core`'s `list.h`, I provide this
`gnrc_pktsnip_t`-specific API to manipulate the packet list. It has the
additional advantage of type safety and that future steps could
encapsulate those operations.

OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,miri64/RIOT,authmillenon/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,authmillenon/RIOT,OTAkeys/RIOT,kaspar030/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT