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).

srcparams.h

@@ -41,6 +41,7 @@ typedef struct {
   int presend_minpoll;
   int iburst;
   int min_stratum;
+  int poll_target;
   unsigned long authkey;
   double max_delay;
   double max_delay_ratio;
@@ -54,6 +55,7 @@ typedef struct {
 #define SRC_DEFAULT_MAXDELAY 16.0
 #define SRC_DEFAULT_MAXDELAYRATIO 16384.0
 #define SRC_DEFAULT_MINSTRATUM 0
+#define SRC_DEFAULT_POLLTARGET 6
 #define INACTIVE_AUTHKEY 0UL
 
 #endif /* GOT_SRCPARAMS_H */