sources: add option to limit selection of unreachable sources

Add maxunreach option to NTP sources and refclocks to specify the
maximum number of polls that the source can stay selected for
synchronization when it is unreachable (i.e. no valid sample was
received in the last 8 polls).

It is an additional requirement to having at least one sample more
recent than the oldest sample of reachable sources.

The default value is 100000. Setting the option to 0 disables selection
of unreachable sources, which matches RFC 5905.

refclock.h

@@ -42,6 +42,7 @@ typedef struct {
   int pps_rate;
   int min_samples;
   int max_samples;
+  int max_unreach;
   int sel_options;
   int max_lock_age;
   int stratum;