sources: add flag that source is active

When source is set as active, it's receiving reachability updates (e.g.
offline NTP sources are not active).

Also add function to count active sources.

refclock.c

@@ -267,6 +267,7 @@ RCL_StartRefclocks(void)
     RCL_Instance inst = &refclocks[i];
 
     SRC_SetSelectable(inst->source);
+    SRC_SetActive(inst->source);
     inst->timeout_id = SCH_AddTimeoutByDelay(0.0, poll_timeout, (void *)inst);
 
     if (inst->lock_ref) {