Temporary change from archive/tar to github.com/dotcloud/tar

This has the new xattrs support. We need this until the xattrs patches
have landed upstream and we can rely on the new go release with it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Add Lgetxattr and Setxattr implementations

These will be used later

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

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Detect file changes to capability bits

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Handle capabilities in tar files

If a file has a security.capability set, we push this to the tar file.
This is important to handle in e.g. layer files or when copying files
to containers, as some distros (e.g. Fedora) use capability bits as
a more finegrained version of setuid bits, and thus if the capabilites
are stripped (and setuid is not set) the binaries will fail to work.

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