[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 ?
>
>:U is already used by bmake for something else- I can't remember what, but
>I checked the man page last night :(

http://www.crufty.net/sjg/blog/freebsd-meta-mode.htm

might provide some interesting background.
It is a more FreeBSD focused (and up to date), coverage of material I
presented at BSDCan a couple of years ago.