syscall: use fcntl F_DUP2FD_CLOEXEC in forkAndExecInChild on dragonfly

syscall: use fcntl F_DUP2FD_CLOEXEC in forkAndExecInChild on dragonfly

Use fcntl(oldfd, F_DUP2FD_CLOEXEC, newfd) to duplicate the file
descriptor and mark is as close-on-exec instead of dup2 & fcntl.

DragonFly BSD implements dup3 like this in libc since version 5.4.

Change-Id: I80c765faa288add8ffb236284c9e8c4f8e6c6769
Reviewed-on: https://go-review.googlesource.com/c/go/+/430535
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jenny Rakoczy <jenny@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Jenny Rakoczy <jenny@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>