Bugfix: Correctly dump directories into tar files

The dump command previously wrote directories in a tar file in a way which
can cause compatibility problems. This caused, for example, 7zip on Windows
to not open tar files containing directories. In addition it was not possible
to dump directories with extended attributes. These compatibility problems
are now corrected.

In addition, a tar file now includes the name of the owner and group of a file.

https://github.com/restic/restic/issues/2319
https://github.com/restic/restic/pull/3039