refclock: add tai option

This option is for indicating to chronyd that the reference clock is
kept in TAI and that chrony should attempt to convert from TAI to UTC by
using the timezone configured by the "leapsectz" directive.

refclock.h

@@ -44,6 +44,7 @@ typedef struct {
   int sel_options;
   int max_lock_age;
   int stratum;
+  int tai;
   uint32_t ref_id;
   uint32_t lock_ref_id;
   double offset;