Simplified NTP kernel interface:

Simplified NTP kernel interface:
- kern.ntp.delta gives the delta to apply (ns)
- kern.ntp.tick_delta is the correction applied in each tick (ns)
- kern.ntp.default_tick_delta is the default correction for each tick (ns)
- kern.ntp.big_delta is the threshold for kern.ntp.delta to use
  10x kern.ntp.default_tick_delta, not the normal value
- kern.ntp.adjust can be used to change the current value of kern.ntp.delta
  relatively.
- kern.ntp.next_leaf_second specifies the time_t of the next leaf second change,
  kern.ntp.insert_leaf_second != 0 means a leaf second is inserted, otherwise
  it is removed

All ntp_* variables are manipulated on CPU #0 with the exception of
set_timeofday. It just sets ntp_delta to 0, which is fine for the moment.