tarsum: include xattr headers in to the checksum

this is to enhance the tarsum algorithm, but _MUST_ be done in lock step
with the same for docker-registry. (PR https://github.com/dotcloud/docker-registry/pull/384 )

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)