archive: Remove unused features

This simplifies that code that calls out to tar by removing support
for now unused features.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Extract code from ExportChanges to addTarFile()

This is the code that takes a normal file and adds it to a TarWriter.
We extract it so that we can share it with Tar().

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

We can use tar.FileInfoHeader to do much of the work.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Implement TarFilter in go, rather than calling out to tar

This uses a plain filepath.Walk + addTarFile to create a tar file,
optionially compressing it with gzip.

Unfortunately go only has gzip compression support, not bzip2 or xz.
However, this is not a regression, as docker currently uses *no*
compression for TarFilter(). The only compression of tarfiles
currently happens in utils/tarsum.go, and that manually does gzip
compression.

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

We no longer call out to tar, so this is not needed.

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

The tar changes made the name of the file in the tar archive
change from  "./test" to "test", update the test to the new name.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Always end directory filenames in tars with /

This matches what tar does, and without it the tarsum created
by the registry will not match the docker one.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3682 from alexlarsson/implement-tar

Implement tar in Go