Bugfixes and enhancements...

Don't enable the oscillator when it is found to be stopped at init time,
just let the first setting of valid time start it. But still report a dead
battery if it's stopped at init time.

Don't force the chip into 24hr mode, just cope with whatever mode it is
already in.

Schedule the clock_settime() callbacks to align the RTC clock to top of
second when setting it.