sys_macosx: make drift removal interval dynamic

sys_macosx: make drift removal interval dynamic

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.