PROG_OVERRIDE_VARS should override default values if specified

The behavior prior to this change would not override default values if set in
bsd.own.mk, or (in the more general case) globally before bsd.progs.mk was
included. This affected bsd.test.mk as well, since it consumes
bsd.progs.mk.

Some examples of this failing behavior are as follows:

• BINMODE defaults to 0555 per bsd.own.mk. 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.
• An application, prog, does not build at WARNS?= 6. Before this change, setting to a lower WARNS value, e.g., WARNS.prog= 3, would have been impossible, requiring that prog be built from another directory, the global WARNS be lowered, or a per-PROG value needing to be set across the board. None of the above workarounds is desirable.

This change unbreaks variables defined in PROG_OVERRIDE_VARS which have
defaults set before bsd.progs.mk is included, by setting them to their
defined values if set on a per-PROG basis.

Reviewed by: asomers
Approved by: emaste (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D19755