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

Follow the implementation used by the other BSDs and account for the
intricacy of having to pass the fds array even though the file
descriptors are returned.

Re-submit of CL 130996 with corrected pipe2 wrapper.

Change-Id: Ie36d8214cba60c4fdb579f18bfc1c1ab3ead3ddc
Reviewed-on: https://go-review.googlesource.com/c/go/+/295372


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