Autodim bugfix (thanks Phil!)

Phil DeGruy noticed that the autodimming didn't work under certain
circumstances.  It seems I introduced this bug when rewriting code to
support alternative ways to drive the IV-18.  Oops!

This bug has been fixed.
New feature:  low battery early warning

When sleeping, if the battery voltage falls below a prespecified level
(new config.h setting).  A warning message ("bad batt") will be
displayed upon waking.  Pressing any button will dismiss the warning
until a bad battery is again while sleeping.