Bugfix: Support non-UTF8 paths as symlink target

Earlier restic versions did not correctly `backup` and `restore` symlinks that
contain a non-UTF8 target. Note that this only affected systems that still use
a non-Unicode encoding for filesystem paths.

The repository format is now extended to add support for such symlinks. Please
note that snapshots must have been created with at least restic version 0.16.0
for `restore` to correctly handle non-UTF8 symlink targets when restoring them.

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