dhclient: support VID 0 (no vlan) decapsulation
VLAN ID 0 is supposed to be interpreted as having no VLAN with a bit of
priority on the side, but the kernel is not able to decapsulate this on
the fly so dhclient needs to take care of it.
Reviewed by: markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D31515