syscall, os: use pipe2 syscall on DragonflyBSD instead of pipe

Follow the implementation used by the other BSDs ith os.Pipe and
syscall.forkExecPipe consisting of a single syscall instead of three.

Change-Id: I602187672f244cbd8faaa3397904d71d15452d9f
Reviewed-on: https://go-review.googlesource.com/130996


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>