Don't shell out to tar for ExportChanges

This changes ExportChanges to use the go tar support so we can
directly create tar layer files. This has several advantages:

* We don't have to create the whiteout files on disk to get them
  added to the layer

* We can later guarantee specific features (such as xattrs) being
  supported by the tar implementation.

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

stat.Rdev and time.* is 32bit on OSX, which breaks cross builds with eg:
cannot use stat.Rdev (type int32) as type uint64 in function argument

We fix this with an extra conversion to uint64.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3292 from alexlarsson/export-changes-manual

Don't shell out to tar for ExportChanges