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-down-pane.c

@@ -29,7 +29,7 @@ int	cmd_down_pane_exec(struct cmd *, struct cmd_ctx *);
 const struct cmd_entry cmd_down_pane_entry = {
 	"down-pane", "downp",
 	CMD_TARGET_WINDOW_USAGE,
-	0, 0,
+	0, "",
 	cmd_target_init,
 	cmd_target_parse,
 	cmd_down_pane_exec,