Adjust the drift removal interval based on the observed offset_sd.
A newly calculated interval goes into effect after the current drift
removal has completed. When offset_sd is high, the interval is increased
resulting in fewer wakeups to adjust the drift offset. At lower values
of offset_sd the drift removal adjustment interval is pinned to 0.5
seconds. The predicted error applied at the start of an adjustment is
based on the remaining time of the drift removal that is currently in
effect. Default drift removal adjustment interval is 4.0 seconds (was
1.0). If not synchronised set interval to maximum of default interval
and current interval. Clamp elapsed drift removal time to
[0, current_drift_removal_interval] to cover clock stepping.
In drivers with periodic drift removal include in the adjustment also a
prediction of the error gained in half of the interval to move the mean
offset of the clock closer to zero. E.g. offset of a stable clock
drifting by 10 ppm should now be closer to 0 +/- 5 microseconds instead
of 5 +/- 5 microseconds.
The darwin kernel implementation of adjtime() does not require the
adjustment to be aligned to a tickadj boundary, and we can apply
adjustments to the nearest microsecond. Rounding is accounted for by
adding any rounding errors back into the offset.
