refclock: add option to filter wrong pulse edges

Add width option to the refclock directive to set expected width of
pulses in a PPS signal. The width adds a limit for the maximum offset
and root distance in order to reject PPS samples from wrong events, e.g.
PHCs which cannot be configured to timestamp only rising of falling
edges.

refclock.h

@@ -49,6 +49,7 @@ typedef struct {
   double delay;
   double precision;
   double max_dispersion;
+  double pulse_width;
 } RefclockParameters;
 
 typedef struct RCL_Instance_Record *RCL_Instance;