cmdmon+client: add selectopts command

This command uses the new source function to modify configured selection
options of an NTP source or reference clock.

pktlength.c

@@ -129,6 +129,7 @@ static const struct request_length request_lengths[] = {
   REQ_LENGTH_ENTRY(select_data, select_data),   /* SELECT_DATA */
   REQ_LENGTH_ENTRY(null, null),                 /* RELOAD_SOURCES */
   REQ_LENGTH_ENTRY(doffset, null),              /* DOFFSET2 */
+  REQ_LENGTH_ENTRY(modify_select_opts, null),   /* MODIFY_SELECTOPTS */
 };
 
 static const uint16_t reply_lengths[] = {