allow an autogroup value of 0 to mean no group. This means you can set

automatically "sticky" (in the traditional sense of the word) windows in
autogroup mode.

Based on an initial diff from Andrew Fresh, thanks!

ok okan@.

group.c

@@ -328,6 +328,9 @@ group_autogroup(struct client_ctx *cc)
 		}
 	}
 
+	if (strncmp("nogroup", group, 7) == 0)
+		return;
+
 	TAILQ_FOREACH(gc, &Groupq, entry) {
 		if (strcmp(shortcut_to_name[gc->shortcut], group) == 0) {
 			_group_add(gc, cc);