Add a configuration option for fullscreen locking

Some people are annoyed to have this new behaviour forced for some
application which use fake fullscreen.

dwm.c

@@ -835,7 +835,7 @@ focusstack(const Arg *arg)
 {
 	Client *c = NULL, *i;
 
-	if (!selmon->sel || selmon->sel->isfullscreen)
+	if (!selmon->sel || (selmon->sel->isfullscreen && lockfullscreen))
 		return;
 	if (arg->i > 0) {
 		for (c = selmon->sel->next; c && !ISVISIBLE(c); c = c->next);