stand: split fg/bg handling up a little further

These can be setup independently, so we should also check them and
initialize each independently. This fixes a pre-existing bug where-in
we may not pickup a bg color specified in the environment if a fg color
wasn't set.

The new version also ensures that we're hooking the color vars properly
if we're using a value that was already there, as the console may need
to adjust if something wants to switch them up again. Otherwise, a
teken.fg_color set in loader could conceivably occur that only changes
the color when you get to the kernel, which could be surprising.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50887