Log Message:
Add support for PROGS (not just PROG) more or less like in the current
NetBSD make library. Bump version (to 20131207, which was when I wrote
this.) Fixes the build of multimedia/bsdav.

Note: this is not a merge from NetBSD HEAD, but a minimal
reimplementation of the feature to get the job done.

These make templates really need to be merged with or replaced by the
latest versions from NetBSD HEAD. Anyone embarking on such a merge
should probably revert this change (which is only to bsd.prog.mk)
before doing so.

No objection after 3+ weeks on tech-pkg.