Move PROGS logic to proper place and remove redundant and unneeded logic.

• bsd.progs.mk is safe to include regardless of PROGS/PROGS_CXX/SCRIPTS being set.
• bsd.progs.mk includes bsd.prog.mk always and will bring in bsd.files.mk and bsd.obj.mk.
• DIRDEPS_BUILD: There's no need for _SKIP_BUILD or _SKIP_STAGING as the PROGS were implicitly being built by the staging dependency anyway. This was also preventing other objects from building if they were not part of the staging sets.
• DIRDEPS_BUILD: This fixes PROGS without bsd.test.mk not staging.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division