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
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272055 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f