Enhancement: Add support for extended attributes on symlinks

Restic now supports extended attributes on symlinks when backing up,
restoring, or FUSE-mounting snapshots. This includes, for example, the
`security.selinux` xattr on Linux distributions that use SELinux.

https://github.com/restic/restic/issues/4375
https://github.com/restic/restic/pull/4379
