By introducing the new 'File_system_registry', we can remove the
knowledge about the actual file-system implementations from the
'Dir_file_system'. Thereby the code becomes more generic, which is
a precondition for using it as the basis for Genode's VFS library.
Issue #999
The random file-system provides an arc4 based urandom implementation
which is needed for OpenSSL.
NOTE: the Arc4random class currently _does not collect enough_ random
bytes!