sources: add option to limit selection by root distance

Add maxdistance directive to set the maximum root distance the sources
are allowed to have to be selected. This is useful to reject NTPv4
sources that are no longer synchronized and report large dispersion.
The default value is 3 seconds.

sources.c

@@ -70,6 +70,7 @@ typedef enum {
   SRC_OK,               /* OK so far, not a final status! */
   SRC_UNSELECTABLE,     /* Has noselect option set */
   SRC_BAD_STATS,        /* Doesn't have valid stats data */
+  SRC_BAD_DISTANCE,     /* Has root distance longer than allowed maximum */
   SRC_WAITS_STATS,      /* Others have bad stats, selection postponed */
   SRC_STALE,            /* Has older samples than others */
   SRC_FALSETICKER,      /* Doesn't agree with others */
@@ -155,6 +156,7 @@ static int selected_source_index; /* Which source index is currently
 /* Number of updates needed to reset the distant status */
 #define DISTANT_PENALTY 32
 
+static double max_distance;
 static double reselect_distance;
 static double stratum_weight;
 static double combine_limit;
@@ -179,6 +181,7 @@ void SRC_Initialise(void) {
   n_sources = 0;
   max_n_sources = 0;
   selected_source_index = INVALID_SOURCE;
+  max_distance = CNF_GetMaxDistance();
   reselect_distance = CNF_GetReselectDistance();
   stratum_weight = CNF_GetStratumWeight();
   combine_limit = CNF_GetCombineLimit();
@@ -653,6 +656,12 @@ SRC_SelectSource(SRC_Instance updated_inst)
       continue;
     }
 
+    /* Require the root distance to be below the allowed maximum */
+    if (si->root_distance > max_distance) {
+      sources[i]->status = SRC_BAD_DISTANCE;
+      continue;
+    }
+
     sources[i]->status = SRC_OK; /* For now */
 
     if (sources[i]->reachability && max_reach_sample_ago < first_sample_ago)
@@ -1216,6 +1225,7 @@ SRC_ReportSource(int index, RPT_SourceReport *report, struct timeval *now)
     switch (src->status) {
       case SRC_UNSELECTABLE:
       case SRC_BAD_STATS:
+      case SRC_BAD_DISTANCE:
       case SRC_STALE:
       case SRC_WAITS_STATS:
         report->state = RPT_UNREACH;