ntp: add support for interleaved symmetric mode

Add xleave option to the peer directive to enable an interleaved mode
compatible with ntpd. This allows peers to exchange transmit timestamps
captured after the actual transmission and significantly improve
the accuracy of the measurements.

srcparams.h

@@ -42,6 +42,7 @@ typedef struct {
   int max_sources;
   int min_samples;
   int max_samples;
+  int interleaved;
   int sel_options;
   uint32_t authkey;
   double max_delay;