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
These device are mandatory for most programs (well, at least null
is required to be present for a POSIX compliant OS, which Noux is
actually not). But for proper shell-script support we will need
them anyway.