refclock: rework update of reachability

Update the reachability register of a refclock source by 1 if a valid
measurement is received by the drivers between source polls, and not
only when it is accumulated to sourcestats, similarly to how
reachability works with NTP sources.

This avoids drops in the reported reachability when a PHC refclock is
dropping samples due to significant changes in the measured delay (e.g.
due to high PCIe load), or a PPS refclock dropping samples due to failed
lock.

refclock_shm.c

@@ -109,6 +109,8 @@ static int shm_poll(RCL_Instance instance)
 
   shm->valid = 0;
 
+  RCL_UpdateReachability(instance);
+
   receive_ts.tv_sec = t.receiveTimeStampSec;
   clock_ts.tv_sec = t.clockTimeStampSec;