Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Add lib80211, a small (but hopefully soon to grow) set of library
routines to interface with net80211.

This is all from the ifconfig program; the duplicate code from ifconfig
will be removed when it starts using this API.

Differential Revision: https://reviews.freebsd.org/D4290