Improved per-digit brightness controls (thanks Justin!)

Justin Hayes suggested adding a reset function to the per-digit
brightness controls to make adjustment easier.

The button code has always been able to interpret simultaneous button
presses, but the interface code has never exploited that feature--
until now!

The per-digit brightness controls (under "cfg disp"/"digt bri") now
understand two additional commands:
  (1) Pressing "set" and "+" simultaneously resets the brightness of
      the current digit to the minimum (default) value.
  (2) Pressing "menu" and "+" simultaneously resets the brightness of
      all digits to the minimum (default) value.