ntp: drop initstepslew NTP implementation

The initstepslew code has its own minimal NTP implementation. Drop the
code, add a new initstepslew mode to the reference updating code and
use regular NTP sources with iburst flag for initstepslew addresses
instead. When an update is made or a source is found unreachable, log a
message, remove the initstepslew sources and switch to normal mode.

This reduces code duplication and makes initstepslew use features
implemented only in the main code like source combining or SO_TIMESTAMP
support.

sources.c

@@ -363,6 +363,12 @@ SRC_UpdateReachability(SRC_Instance inst, int reachable)
     /* Try to select a better source */
     SRC_SelectSource(NULL);
   }
+
+  /* End special reference mode on last reachability update from iburst */
+  if (REF_GetMode() != REF_ModeNormal &&
+      inst->reachability_size >= REACH_BITS - 1) {
+    REF_SetUnsynchronised();
+  }
 }
 
 /* ================================================== */