reference: allow clock adjustments without updating reference

Add support for accumulating frequency and time offset without changing
the reference parameters and calling the local parameter change
handlers.

This will allow an unsynchronized source to operate below other sources
in order to stabilize the clock.

reference.h

@@ -162,6 +162,10 @@ extern void REF_SetManualReference
 extern void
 REF_SetUnsynchronised(void);
 
+/* Make a small correction of the clock without updating the reference
+   parameters and calling the clock change handlers */
+extern int REF_AdjustReference(double offset, double frequency);
+
 /* Announce a leap second before the full reference update */
 extern void REF_UpdateLeapStatus(NTP_Leap leap);