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.