Add DEBUG_FLAGS to PROG_VARS and STRIP to PROG_OVERRIDE_VARS

This will allow the variables [*] to be overridden on a per-PROG basis,
which is useful when controlling "stripping" behavior for some tests
that require debug symbols or to be unstripped

DEBUG_FLAGS (similar to CFLAGS) supports appending, whereas STRIP is
an override

*: Due to how STRIP is defined in bsd.own.mk (in addition to
bsd.lib.mk and bsd.prog.mk), and the fact that bsd.test.mk pulls in
bsd.own.mk first, overriding STRIP doesn't work today.

A follow up commit is pending to "rectify" this after additional
testing is done.

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