ntp: add burst option

When the burst option is specified in the server/pool directive and the
current poll is longer than the minimum poll, initiate on each poll a
burst with 1 good sample and 2 or 4 total samples according to the
difference between the current and minimum poll.

srcparams.h

@@ -35,6 +35,7 @@ typedef struct {
   int online;
   int auto_offline;
   int presend_minpoll;
+  int burst;
   int iburst;
   int min_stratum;
   int poll_target;