Now it's not very different from syscall_dragonfly.go in
golang.org/x/sys/unix repository.

Change-Id: I8dfd22e1ebce9dc2cc71ab9ab7f0c92d93b2b762
Reviewed-on: https://go-review.googlesource.com/41835
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>