cmdmon: refactor handling of sources report

The NSR_ReportSource() and RCL_ReportSource() functions assume that the
provided report already has some data prefilled by SRC_ReportSource()
and it's assumed these functions cannot fail.

Change them to accept the required data (refid and IP address) as
a parameter, remove unneeded parameters, and return an error status
(if the refid/address doesn't exist) to be handled in cmdmon
handle_source_data(). Also, catch unexpected values of the source state
and mode to make chronyc report an error instead of incorrect data.

stubs.c

@@ -106,10 +106,10 @@ RCL_StartRefclocks(void)
 {
 }
 
-void
-RCL_ReportSource(RPT_SourceReport *report, struct timespec *now)
+int
+RCL_ReportSource(uint32_t ref_id, RPT_SourceReport *report)
 {
-  memset(report, 0, sizeof (*report));
+  return 0;
 }
 
 int