Add EZP_INI_DIRECTORY_PERMISSION constant

This pull request fixes two issues I had with permissions in var folder:

* EZP_INI_FILE_PERMISSION was used to create the var/autoload directory as well as files inside of it. If we want our INI caches and autoload files to have 0660 permissions, it is not possible to enter the var/autoload directory by eZ Publish as no execute bit was set on it.

* eZINI class uses hardcoded 0777 value for creating /var/cache/ini folder

With this pull request, both of those issues are resolved by using a new constant in config.php