implement reception of "VLAN 0 priority tagged" packets.

implement reception of "VLAN 0 priority tagged" packets.

according to 802.1Q, vlan 0 on the wire is special and should be
interpreted as if it was a packet received on the parent interface,
but you get the packet priority information encoded in the vlan
header.

historically we drop vlan tagged packets that don't have a vlan
interface configured for the received tag number. historically we
have deviated from 802.1Q by allowing for the configuration of a
vlan subinterface with the vnetid "unset". this works just like any
other vlan interface, but it uses tag 0 on the wire. however, if
you're in a situation where you're receiving vlan tagged 0 packets
that really are part of the same layer 2 ethernet domain as the
parent inteface, this doesnt work well.

landry@ is in such a situation at work where the network is sending
his OpenBSD boxes packets with VLAN tag 0. sometimes. most of the
time the packets are untagged, which is expected, but sometimes
they have a VLAN header set. this causes problems, particularly
with arp.

this diff does the smallest possible change to enable reception of
these vlan 0 priority tagged packets. if an "unset" vlan interface
is not configured on the parent, then vlan 0 tagged packets get
their header stripped and continue stack processing as if they didnt
have the tag at all.

landry has been running this for months.
ok sthen@ claudio@