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-set-buffer.c

@@ -31,7 +31,7 @@ int	cmd_set_buffer_exec(struct cmd *, struct cmd_ctx *);
 const struct cmd_entry cmd_set_buffer_entry = {
 	"set-buffer", "setb",
 	CMD_BUFFER_SESSION_USAGE " data",
-	CMD_ARG1, 0,
+	CMD_ARG1, "",
 	cmd_buffer_init,
 	cmd_buffer_parse,
 	cmd_set_buffer_exec,