allow interfaces to be marked as externally controlled
allow interfaces to be marked as externally controlled

jlyo/netcfg,rubyists/netcfg2,GNU-Pony/netcfg,iphitus/netcfg-updates,Finkregh/netcfg-vlan,GNU-Pony/netcfg,Finkregh/netcfg-vlan,somenxavier/netcfg,somenxavier/netcfg