[ticket/11334] Properly generate controller URL with helper url() method

Currently the url() method generates URLs like ./forum/app.php/path/to/page. Instead, until the paths issue gets fixed, we have to use app.php?controller=path/to/page. This was not changed back when the decision was made, so here it is now.

http://tracker.phpbb.com/browse/PHPBB3-11334