Fix issues on "Set interface(s) IP address" option
Fix issues on "Set interface(s) IP address" option

- Avoid duplicate gateway entries
- Fix checking if a default gw already exists
- Set IPv6 to none when user choose it
- Set gateway on interface

ch1c4um/pfsense,NOYB/pfsense,NOYB/pfsense,dennypage/pfsense,jxmx/pfsense,NewEraCracker/pfsense,NewEraCracker/pfsense,ptorsten/pfsense,ptorsten/pfsense,dennypage/pfsense,BlackstarGroup/pfsense,BlackstarGroup/pfsense,phil-davis/pfsense,pfsense/pfsense,jxmx/pfsense,brunostein/pfsense,jxmx/pfsense,NOYB/pfsense,pfsense/pfsense,NewEraCracker/pfsense,pfsense/pfsense,phil-davis/pfsense,brunostein/pfsense,dennypage/pfsense,ch1c4um/pfsense,phil-davis/pfsense,ch1c4um/pfsense,dennypage/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,BlackstarGroup/pfsense,brunostein/pfsense,BlackstarGroup/pfsense,pfsense/pfsense,phil-davis/pfsense