Allow bsd.compat.mk to be reliably included outside Makefile.inc1.

Replace explicit TARGET_* variables with COMPAT_* versions defined based
on where the file is being included.

Also, require that bsd.compat.mk be included directly. It's not going to
be widely used so always loading it in bsd.prog.mk doesn't make sense.
Instead users can include it directly.

Reviewed by: imp, bdrewery (prior revision)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D22059