Finer control of per-digit brightness

The per-digit brightness controls ("cfg disp"/ "digt bri"), now
provide much finer control.  With the finer control, the experimental
brightness gradient correction is not necessary and has been removed.

The "date fmt" and "time fmt" entries are now in reverse order to
match the order of "set time" and "set date".  This also puts
"time fmt" next the other time controls, "set dst " and "set zone".

The README and CREDITS files have been updated to reflect new
features.  The new auto-off feature is now better commented in the
source code.