dir: Refuse to export .desktop files with suspicious uses of @@ tokens

This is either a malicious/compromised app trying to do an attack, or
a mistake that will break handling of %f, %u and so on. Either way,
if we refuse to export the .desktop file, resulting in installation
failing, then it makes the rejection more obvious than quietly
removing the magic tokens.

Signed-off-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 46b3ede5241561c7d588951048c687c5075a3eac)