Add a way to mark environment variables as "hidden" so they can be used

by tmux but are not passed into the environment of new panes.

cmd-respawn-pane.c

@@ -71,7 +71,7 @@ cmd_respawn_pane_exec(struct cmd *self, struct cmdq_item *item)
 
 	add = args_first_value(args, 'e', &value);
 	while (add != NULL) {
-		environ_put(sc.environ, add);
+		environ_put(sc.environ, add, 0);
 		add = args_next_value(&value);
 	}