Add zend-opcache module recipe

Also fix a bug when installing PHP: make sure PHP config dir exists
`fpm` won't create `conf_dir`. It installs `php5-fpm` (on Ubuntu), which creates `conf_dir`. I'm not sure if I want to support changing `conf_dir`. Maybe it should be `set` instead of `default`. Are you sure having `fpm` before `module_opcache` doesn't solve the problem? I don't want to believe that the package doesn't create the file it's supposed to create...