The behavior prior to this change would not override default values if set in
bsd.own.mk. For example: BINMODE defaults to 0555. If someone wanted to set
the BINMODE to NOBINMODE (0444) for prog, for example, like
BINMODE.prog= ${NOBINMODE}, bsd.progs.mk would not honor the per-prog
setting.
This change unbreaks variables defined in PROG_OVERRIDE_VARS which have
defaults set before bsd.progs.mk is included.
MFC after: 1 momth