dir: Reserve the whole @@ prefix

If we add new features analogous to file forwarding later, we might
find that we need a different magic token. Let's reserve the whole
@@* namespace so we can call it @@something-else.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 1e7e8fdb24b51078f4c48e0711e24a14930ba1f0)