local: reset daemon after unexpected time jump

Add a new change type and use it when an unexpected time jump is
detected in the scheduler to reset reference times, offset and slewing,
NCR instances (with their polling interval), synchronization status, and
drop all sourcestats, manual, refclock and RTC samples.

This should make the recovery more graceful if the estimated jump has a
large error (e.g. select didn't timeout, or after system suspend).

rtc_linux.c

@@ -273,6 +273,11 @@ slew_samples
   double delta_time;
   double old_seconds_fast, old_gain_rate;
 
+  if (change_type == LCL_ChangeUnknownStep) {
+    /* Drop all samples. */
+    n_samples = 0;
+  }
+
   for (i=0; i<n_samples; i++) {
     UTI_AdjustTimeval(system_times + i, cooked, system_times + i, &delta_time,
         dfreq, doffset);