cmdmon: add reset command

The reset command drops all measurements and switches the reference to
the unsynchronised state. This command can help chronyd with recovery
when the measurements are known to be no longer valid or accurate, e.g.
due to moving the computer to a different network, or resuming the
computer from a low-power state (which resets the system clock).

candm.h

@@ -103,7 +103,8 @@
 #define REQ_ONOFFLINE 63
 #define REQ_ADD_SOURCE 64
 #define REQ_NTP_SOURCE_NAME 65
-#define N_REQUEST_TYPES 66
+#define REQ_RESET 66
+#define N_REQUEST_TYPES 67
 
 /* Structure used to exchange timespecs independent of time_t size */
 typedef struct {