archive/tar: Add support for extended attributes

[gopherbot]

by alexander.larsson:

archive/tar currently has no support for extended attributes (xattrs). Docker is using
tar files and needs to support extended attributes.

Both Gnu tar and star uses the "SCHILY.xattr.<xattr-name>" pax header
inside a posix format archive for this.

[gopherbot]

Comment 1 by alexander.larsson:

Added a patch here:
https://golang.org/cl/54570043/

[gopherbot]

Comment 2 by alexander.larsson:

For futher info, here is how this would be used in docker:
moby/moby#3845

[gopherbot]

Comment 3 by PaulTag:

Sorely needed!

[davecheney]

Comment 4:

Labels changed: added release-none, repo-main.

Status changed to Started.

[dsymonds]

Comment 5:

This issue was closed by revision 87d58f4.

Status changed to Fixed.