New display options and adjustments

Under "cfg regn"/"digt fmt", users may select between adjusted numbers
and zero-padded numbers and between representations of nine.

A new GPS-lock indicator may be displayed when the AM/PM indicator is
unneeded or the DST status is hidden.

There is an new experimental display-gradient correction included
under "cfg disp"/"digt bri".

The Adafruit Ultimate GPS seems to output an incorrect NMEA RMC record
immediately after turning on.  This caused the clock to start with an
incorrect time after waking sleep.  The GPS time code now disregards
the first good NMEA RMC record, and sets time after parsing the second
good record.