Introduce offset correction rate

We want to correct the offset quickly, but we also want to keep the
frequency error caused by the correction itself low.

Define correction rate as the area of the region bounded by the graph of
offset corrected in time. Set the rate so that the time needed to correct
an offset equal to the current sourcestats stddev will be equal to the
update interval (assuming linear adjustment). The offset and the
time needed to make the correction are inversely proportional.

This is only a suggestion and it's up to the system driver how the
adjustment will be executed.

sys_netbsd.c

@@ -199,7 +199,7 @@ stop_adjust(void)
    slew backwards */
 
 static void
-accrue_offset(double offset)
+accrue_offset(double offset, double corr_rate)
 {
   stop_adjust();
   offset_register += offset;