Pull up fixes to allow building tests along scripts and data files.

Pull up fixes to allow building tests along scripts and data files.

MFC of the following into stable/10:
- r257095 Allow mixing bsd.files.mk with bsd.subdir.mk.
- r258095 Allow this (bsd.progs.mk) to work with fmake.
- r258330 Need to also test for defined(${v}_${PROG}) in bsd.progs.mk.
- r259209 Make bsd.progs.mk work in directories with SCRIPTS but no PROGS.

This is all 'make tinderbox' clean as run on ref10-amd64.

git-svn-id: svn://svn.freebsd.org/base/stable/10@260005 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f