Make smtp_queue_fs::scan_folder rely on openat like everywhere else

[Ekleog]

It's currently the only place where we use a non-openat version, except when opening the initial directory. This means that there's a race condition if a directory is being replaced between queue creation and folder scan; and things probably will go quite wrong.

Blocked on https://github.com/BurntSushi/walkdir/issues/136 that's been redirected to https://github.com/BurntSushi/walkdir/issues/23 for initial openat support