[ticket/9801] Allow display of page outside board directory on viewonline

Until now, the preg_match didn't take into account that a page might be
outside the board directory. After this change, the regular expression will
allow preceding dots and slashes. Paths like ../../test.php will be allowed.
The page will be unknown in viewonline.php and it will revert to index.php as
current page. However, the core.viewonline_overwrite_location event will
receive proper info of the page the user is on right now allowing a listener
to change the info that is being displayed.

PHPBB3-9801

Ticket: https://tracker.phpbb.com/browse/PHPBB3-9801