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