The new ifconfig options 'ifvlan', '+ifvlans' and '-ifvlans' allow the
vlan access list of a bridge port to be configured.
Incoming tagged frames will be dropped if the incoming vlan tag isn't
in the port's access list.
Outgoing frames will be dropped if the outgoing vlan ID isn't in the
port's access list (e.g., for BUM traffic).
This is only enabled if the port's pvid is set to something other than
zero, which is treated as the flag to enable vlan filtering.