Defer focusing on sticky windows on switching tags

focus(NULL) is called when switching to a new tag or monitor.  I don't
want sticky windows to get first focus in this situation, hence this
code.  Shamelessly stolen from
https://github.com/LukeSmithxyz/dwm/issues/152.

dwm.c

@@ -906,8 +906,14 @@ expose(XEvent *e)
 void
 focus(Client *c)
 {
-	if (!c || !ISVISIBLE(c))
-		for (c = selmon->stack; c && !ISVISIBLE(c); c = c->snext);
+	if (!c || !ISVISIBLE(c)) {
+		for (c = selmon->stack; c &&
+					 (!ISVISIBLE(c) || (c->issticky && !selmon->sel->issticky));
+				 c = c->snext);
+		if (!c) /* No windows found; check for available stickies */
+			for (c = selmon->stack; c && !ISVISIBLE(c); c = c->snext);
+	}
+
 	if (selmon->sel && selmon->sel != c)
 		unfocus(selmon->sel, 0);
 	if (c) {