Add Fgetxattr, Flistxattr, Fremovexattr and Fsetxattr on Darwin. Also
add a corresponding test.

Updates golang/go#26832

Change-Id: Id75bfce90ccc024b567a7b066a9188a615b9eec4
Reviewed-on: https://go-review.googlesource.com/128537
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>