MFC r297282,r297456,r298012,r298013,r298014:

r297282 (by bdrewery):

We don't have a CPPFLAGS, COPTS or CPUFLAGS.

r297456 (by bdrewery):

We don't support DPLIBS.

r298012:

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

r298013:

Commit documentation change for r298012

Requested by: bdrewery

r298014:

Regenerate the list of bsd.progs.mk supported variables

Prefix with dashes (unordered list) and put one variable on each
line (to avoid future conflicts)

Done via the following one-liner:

sh -c 'for i in $(make -C tests/sys/aio PROG=foo -VPROG_VARS:O); do printf "\t\t- $i\n"; done'