Allow overwrite in archive.Untar

This will *by default* overwrite all non-directory files in `Untar`, to mimic the tar cli:

```
$ mkdir foo
$ echo overwrite me > foo/nomercy
$ touch nomercy
$ tar -f archive.tar -c nomercy
$ cd foo && tar -x -f ../archive.tar
$ cat nomercy
$
```

All tests pass for me, I couldn't see bad side effects in image layers from this. Let me know if you have concerns.

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)