[ticket/11824] Add option for mod_rewrite

This option affects the output of the `phpbb_controller_helper::url()` method. When the option is enabled, paths returned by the `url()` method do not contain `app.php`. Otherwise, they do. (`./app.php/route` vs `./route`)

When mod_rewrite is disabled, the option is disabled and cannot be enabled. When mod_rewrite is enabled can be either enabled or disabled. When we can't determine whether or not mod_rewrite is enabled (or even available; i.e. non-apache server or PHP is installed as CGI) we allow the option to be changed but we show a warning that links could be broken if enabled when mod_rewrite is unavailable.

NOTE: This was tested by me but @nickvergessen wants it tested against his extension. This should be done prior to merging this.

http://tracker.phpbb.com/browse/PHPBB3-11824