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).

cmdparse.h

@@ -45,7 +45,8 @@ typedef enum {
   CPS_BadMaxdelayratio,
   CPS_BadMaxdelay,
   CPS_BadKey,
-  CPS_BadMinstratum
+  CPS_BadMinstratum,
+  CPS_BadPolltarget
 } CPS_Status;
 
 typedef struct {