Extract device major/minor number on Darwin and set Devmajor and
Devminor in FileInfoHeader. Code based on the Major/Minor functions for
Darwin in golang.org/x/sys/unix.

Change-Id: I51b65f607bfa2e6b177b8b66e2b246b771367b84
Reviewed-on: https://go-review.googlesource.com/60850Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>