Ignore special dir in clean_stale_files

Signed-off-by: Florian Pritz <bluewind@xinu.at>
mfile->delete_id: Check for file before trying to unlink

file->cron unlinks the file and then deletes the hash which causes an
error in mfile->delete_id because the file no longer exists.

Signed-off-by: Florian Pritz <bluewind@xinu.at>