Bugfix: Support serving a `restic mount` of a Windows system via Samba

A repository mounted using `restic mount` on a POSIX system could not use
Samba to serve files from backups of Windows systems, while backups of
non-Windows systems could be served successfully. This has now been fixed.

https://github.com/restic/restic/issues/2034
https://github.com/restic/restic/issues/4382
https://github.com/restic/restic/pull/21784
