refclock: add local option

Add "local" option to specify that the reference clock is an
unsynchronized clock which is more stable than the system clock (e.g.
TCXO, OCXO, or atomic clock) and it should be used as a local standard
to stabilize the system clock.

Handle the local refclock as a PPS refclock locked to itself which gives
the unsynchronized status to be ignored in the source selection. Wait
for the refclock to get at least minsamples samples and adjust the clock
directly to follow changes in the refclock's sourcestats frequency and
offset.

There should be at most one refclock specified with this option.

refclock.h

@@ -37,6 +37,7 @@ typedef struct {
   int driver_poll;
   int poll;
   int filter_length;
+  int local;
   int pps_forced;
   int pps_rate;
   int min_samples;