sys/net: add DOT1Q_VID_{MIN,MAX}

add a new typedef in if_vlan_var.h to represent the lowest (1) and
highest (4095) valid VLAN IDs in a .1q tag.

change if_vlan to use DOT1Q_VID_MAX instead of a magic number.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D49837

(cherry picked from commit c59492025073bd57c7dc3bb5bd843ab0a1391c7a)