periodically save system time to hardware time-of-day clock

This is done in kern_ntptime, perhaps not the best place.
This is done using resettodr().
Some features:
- make save period configurable via tunable and sysctl
- period of zero disables saving, setting a non-zero period re-enables
  it or reschedules it
- do saving only if system clock is ntp-synchronized
- save on shutdown

Discussed with:	des, Peter Jeremy <peterjeremy@acm.org>
X-Maybe:		save time near seconds boundary for better precision
MFC after:		2 weeks

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