Don't make nested definition for range_seg_cache.

Reported by:	ian
MFC after:	1 week
X-MFC-With:	r272506
Make kevent(2) periodic timer events more reliably periodic.  The event
callout is now scheduled using the C_ABSOLUTE flag, and the absolute time
of each event is calculated as the time the previous event was scheduled
for plus the interval.  This ensures that latency in processing a given
event doesn't perturb the arrival time of any subsequent events.

Reviewed by:	jhb