Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Remove the old depend (mkdep) code and make FAST_DEPEND the one true way.

Reviewed by: emaste, hselasky (partial), brooks (brief)
Discussed on: arch@
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D5742

Add a comment that shows how to limit the build to the specific list of arches.

Add missing ;;

Utility script to build specific parts of the source tree on all arches