ntp: allow presend of zero

Don't use zero as a special value for disabled and change the default
presend to a value larger than any valid poll.

srcparams.h

@@ -54,7 +54,7 @@ typedef struct {
 #define SRC_DEFAULT_PORT 123
 #define SRC_DEFAULT_MINPOLL 6
 #define SRC_DEFAULT_MAXPOLL 10
-#define SRC_DEFAULT_PRESEND_MINPOLL 0
+#define SRC_DEFAULT_PRESEND_MINPOLL 100
 #define SRC_DEFAULT_MAXDELAY 3.0
 #define SRC_DEFAULT_MAXDELAYRATIO 0.0
 #define SRC_DEFAULT_MAXDELAYDEVRATIO 10.0