tarfile: pax header and xattr support

tar archive have additional headers that store file data like security
capabilities and ACLs. These values can be packed structures, not utf8
encoded.
Python's ( < v3.3) tarfile does not support these xattr headers, and has
encoding issues on the values stored. This change includes using a
vendored version of python-2.7.6's tarfile.py, with changes to support
this. While ugly to do it this way, I do see another way to cleanly override _only_ the tarfile.TarInfo._proc_pax method in a separate module.

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)