Bugfix: Support non-UTF8 paths as symlink target

Restic versions before 0.16.0 did not correctly backup and restore symlinks
that contain a non-UTF8 target. Note that this only affects system that still
use a non-Unicode encoding for filesystem paths.

We have extended the repository format to add support for such symlinks. Please
note that at least restic version 0.16.0 must be used for both backup and
restore to correctly handle non-UTF8 symlink targets.

https://github.com/restic/restic/issues/3311
https://github.com/restic/restic/pull/3802