ntp: limit number of pool sources

A new option can be now used in the pool directive: maxsources sets the
maximum number of sources that can be used from the pool, the default
value is 4.

On start, when the pool name is resolved, chronyd will add up to 16
sources, one for each resolved address. When the number of sources from
which at least one valid reply was received reaches maxsources, the
other sources will be removed.

cmdparse.h

@@ -44,7 +44,8 @@ typedef enum {
   CPS_BadKey,
   CPS_BadMinstratum,
   CPS_BadPolltarget,
-  CPS_BadVersion
+  CPS_BadVersion,
+  CPS_BadMaxsources,
 } CPS_Status;
 
 typedef struct {