Give permission options for cache folder

Some people (including myself) don't like to give 0777 permissions to files if it's not necessary. Currently, getting phpBB to use 0775 permissions for the cache is only possible by editing core code. It would be good to have this built in as a config option to avoid this.

See discussion here:
http://www.phpbb.com/community/viewtopic.php?f=46&t=2145003