[CFT/RFC]: refactor bsd.prog.mk to understand multiple programs instead of a singular program

[Simon J. Gerraty]

>In particular, why cannot the ':L' and ':U' support be added ?

Because they already exist - with different meanings.
They were added to NetBSD make over 10 years ago, from the OSF version
of pmake.

In several areas the behavior of bmake has been changed to make it a
drop in replacement for FreeBSD, but the above (not used at all in the
FreeBSD base) are easier dealt with the other way.  The :tl and :tu
equivalents were added to FreeBSD make a while back to ease the
transition.