Extracted eZDFSFileHandlerDFSBackendInterface

> Implements [EZP-22966](https://jira.ez.no/browse/EZP-22966)

Adds an `eZDFSFileHandlerDFSBackendInterface`, implemented by the existing `eZDFSFileHandlerDFSBackend`.

Also changed `dfscleanup.php` so that it is backend independent!
- added a new `getFilesList()` method that lists files from a backend
- moved DFS sanity check within the backend, and use exception handling to detect that NFS isn't available anymore