unix: add Dup3 on dragonfly

Use the same implementation as on freebsd which is also what the
dragonfly libc uses.

Change-Id: I0ed513ae15fcb6dac77b2fc76f662723d66b75c6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/636435
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>