ntp: add client support for experimental extension field

Add "extfield F323" option to include the new extension field in
requests. If the server responds with this field, use the root
delay/dispersion and monotonic timestamp. Accumulate changes in the
offset between the monotonic and real-time receive timestamps and use
it for the correction of previous offsets in sourcestats. In the
interleaved mode, cancel out the latest change in the offset in
timestamps of the previous request and response, which were captured
before the change actually happened.

srcparams.h

@@ -55,6 +55,7 @@ typedef struct {
   int nts;
   int nts_port;
   int copy;
+  int ext_fields;
   uint32_t authkey;
   uint32_t cert_set;
   double max_delay;