ntp: don't stop online burst when sending fails

Don't stop online burst for unreachable sources until sending succeeds.
This is mainly useful with iburst when chronyd is started before the
network is configured.

sources.c

@@ -1167,6 +1167,14 @@ SRC_IsSyncPeer(SRC_Instance inst)
 
 /* ================================================== */
 
+int
+SRC_IsReachable(SRC_Instance inst)
+{
+  return inst->reachability != 0;
+}
+
+/* ================================================== */
+
 int
 SRC_ReadNumberOfSources(void)
 {