archive: Extract createTarFile() from ApplyLayer

This way we can reuse it for Untar()

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Implement UnTar via archive/tar

This replaces the shelling out to tar with a reimplementation of untar
based on the archive/tar code and the pre-existing code from ApplyLayer
to create real files from tar headers.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)