ifconfig: Improve VLAN identifier parsing

VLAN identifier 0xFFF is reserved. It must not be configured or
transmitted.

Also validate during parsing to prevent potential integer overflow.

Reviewed by: network, melifaro
Fixes: c7cffd65c5d85 Add support for stacked VLANs (IEEE 802.1ad, AKA Q-in-Q)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39282