Based on the tests in github.com/pkg/xattr

Change-Id: I3af73ad538877da042602708248063c218260a2e
Reviewed-on: https://go-review.googlesource.com/114135
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>