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

(cherry picked from commit abf5bff71d38da3c797a3b6decb426c375cc0f8f)