reference: make driftfile update interval configurable

Add interval option to the driftfile directive to specify the minimum
interval between updates of the recorded frequency offset and skew.

rtc.c

@@ -81,8 +81,9 @@ get_driftfile_time(void)
 {
   struct stat buf;
   char *drift_file;
+  int interval;
 
-  drift_file = CNF_GetDriftFile();
+  drift_file = CNF_GetDriftFile(&interval);
   if (!drift_file)
     return 0;