Get rid of the ugly CMD_CHFLAG macro and use a const string (eg "dDU") in the

command entry structs and a couple of functions to check/set the flags.

cmd-send-prefix.c

@@ -29,7 +29,7 @@ int	cmd_send_prefix_exec(struct cmd *, struct cmd_ctx *);
 const struct cmd_entry cmd_send_prefix_entry = {
 	"send-prefix", NULL,
 	CMD_TARGET_PANE_USAGE,
-	0, 0,
+	0, "",
 	cmd_target_init,
 	cmd_target_parse,
 	cmd_send_prefix_exec,