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