Intelligent PVID config error detector

[troglobit]

In #349 the issue of not setting PVID on a bridge-port was discussed briefly. We talked about this in the team a bit, ever so briefly, and agreed that the best approach is to have some sort of clever helper in the CLI.

admin@example:/config/> set interface e1 bridge-port pvid 1
admin@example:/config/> leave
Warning: bridge port 'e1' is an untagged member of bridge 'br0' in VLAN 2, but the PVID is 1.
Select action (Fix/Ignore/Manual): __