Base poll adjustment on number of sourcestats samples

Instead of following skew changes, adjust polling interval so that the
number of measurements used in the regression algorithm remains close to
a target value. It can be configured with a new polltarget option
(6 by default).

cmdmon.c

@@ -1251,6 +1251,7 @@ handle_add_source(NTP_Source_Type type, CMD_Request *rx_message, CMD_Reply *tx_m
 
  /* not transmitted in cmdmon protocol yet */
   params.min_stratum = SRC_DEFAULT_MINSTRATUM;       
+  params.poll_target = SRC_DEFAULT_POLLTARGET;
 
   status = NSR_AddSource(&rem_addr, type, &params);
   switch (status) {