smooth: add option to smooth out only leap seconds

The leaponly option can be used to enable a mode where only leap seconds
are smoothed out and normal offset/frequency changes are ignored. This
is useful to make the interval in which a leap second is smoothed out
constant and allow an NTP client to use multiple leap smearing servers
safely.

local.c

@@ -564,6 +564,9 @@ LCL_NotifyLeap(int leap)
   LCL_ReadRawTime(&raw);
   LCL_CookTime(&raw, &cooked, NULL);
 
+  /* Smooth the leap second out */
+  SMT_Leap(&cooked, leap);
+
   /* Dispatch to all handlers as if the clock was stepped */
   invoke_parameter_change_handlers(&raw, &cooked, 0.0, -leap, LCL_ChangeStep);
 }