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 will be cited)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: version the addition of xattrs

Now that TarSum can be versioned, move the addition of xattr headers in
to the next version of TarSum

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #5977 from vbatts/vbatts-tarsum_xattrs

tarsum: include xattr headers in to the checksum
Improve ValidateContextDirectory error handling.

- Errors sent to the walker callback functions were ignored. This meant that
  one could get a panic when calling methods on a nil FileInfo object. For
  example when the file did not exists any more.

- Lstat calls inside walker callback are reduntant because walker already calls
  Lstat and passes the result to the callback.

- Error returned from filepath.Rel() can never be EACCES because it compares
  strings and does not care about actual files.

- If Matched() returns error then ValidateContextDirectory() must return error.
  Currently it still kept walking although the outcome was already known. 

- Function will now fail in case of unknown error(not EACCES nor ENOENT).
  Previous implementation did not make a clear decision about this (but 
  panicked because of the issues above).

Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com> (github: tonistiigi)
Merge pull request #8026 from tonistiigi/validate-context-error-handling

Improve ValidateContextDirectory error handling.