sys/net: move DOT1Q_VID_* constants to ethernet.h

These are generally useful to anything dealing with 802.1q and aren't
specific to if_vlan, so move them to <net/ethernet.h> and remove the
_KERNEL gate.

While here, document what they actually mean.

Reviewed by: kp, des
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50570