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when the measurements are known to be no longer valid or accurate, e.g.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).

pktlength.c

@@ -123,6 +123,7 @@ static const struct request_length request_lengths[] = {
   REQ_LENGTH_ENTRY(ntp_source, null),           /* ADD_SOURCE */
   REQ_LENGTH_ENTRY(ntp_source_name,
                    ntp_source_name),            /* NTP_SOURCE_NAME */
+  REQ_LENGTH_ENTRY(null, null),                 /* RESET */
 };
 
 static const uint16_t reply_lengths[] = {