ntp: add server time smoothing

Time smoothing determines an offset that needs to be applied to the
cooked time to make it smooth for external observers. Observed offset
and frequency change slowly and there are no discontinuities. This can
be used on an NTP server to make it easier for the clients to track the
time and keep their clocks close together even when large offset or
frequency corrections are applied to the server's clock (e.g. after
being offline for longer time).

Accumulated offset and frequency are smoothed out in three stages. In
the first stage, the frequency is changed at a constant rate (wander) up
to a maximum, in the second stage the frequency stays at the maximum for
as long as needed and in the third stage the frequency is brought back
to zero.

Time smoothing is configured by the smoothtime directive. It takes two
arguments, maximum frequency offset and maximum wander. It's disabled by
default.

logging.h

@@ -100,7 +100,8 @@ typedef enum {
   LOGF_SysWinnt,
   LOGF_TempComp,
   LOGF_RtcLinux,
-  LOGF_Refclock
+  LOGF_Refclock,
+  LOGF_Smooth,
 } LOG_Facility;
 
 /* Init function */