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.

candm.h

@@ -246,6 +246,7 @@ typedef struct {
 #define REQ_ADDSRC_NOSELECT 0x10
 #define REQ_ADDSRC_TRUST 0x20
 #define REQ_ADDSRC_REQUIRE 0x40
+#define REQ_ADDSRC_INTERLEAVED 0x80
 
 typedef struct {
   IPAddr ip_addr;