MFC r272055:

MFC r272055:

  Improve bsd.progs.mk compatibility with bsd.prog.mk

  1. Do not install FILES/SCRIPTS multiple times if PROGS is specified; this is
     already handled via bsd.prog.mk when it's called recursively (PR: 191055,
     191955).
  2. Some variables, like BINDIR and PROGNAME, default to a value if unset
     whereas others get appended to, like CFLAGS. Add support for the former case
     (PR: 191056)
  3. Make "checkdpadd" and "clean" available targets for recursive execution.

  Reviewed by: marcel, sjg
  Phabric: D822
  PR: 191055, 191056, 191955
  Sponsored by: EMC / Isilon Storage Division

git-svn-id: svn://svn.freebsd.org/base/stable/10@273501 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f