Getting rid of register_shutdown_function() in cron.php to prevent path disclosure (reported by lacton)

This issue was first reported by lacton via the security tracker.

In certain conditions, phpBB is exposing the full path of cron.php. ie apache access log shows requests:
"GET /var/www/jadephpbb/httpdocs/cron.php?cron_type=tidy_search HTTP/1.1" 404 304 "https://forums.jadeworld.com/viewtopic.php?f=9&t=1206&start=0"

Support topic: http://www.phpbb.com/community/viewtopic.php?f=46&t=2121664