acpiout(4): Work around firmware that doesn't like some brightnesses.

Instead of just asking for cur - 5 or cur + 5, repeatedly ask for
that increment, check whether we actually made progress in that
direction, and if not keep going with another increment, until we hit
the bounds of brightness levels.

I can't find anything in the ACPI spec about this, but my laptop
seems to have trouble with certain levels: 15, 75, 85, 95.  It goes
in all other increments of 5 from 5 to 100, just not those ones --
acts as if the change just never happened, so with the old logic the
brightness up/down would get stuck unable to move in either
direction.

This should have no impact on machines where the first increment
actually takes.

(riastradh)