syscall: use fcntl with F_DUP2FD_CLOEXEC in forkAndExecInChild on FreeBSD

syscall: use fcntl with F_DUP2FD_CLOEXEC in forkAndExecInChild on FreeBSD

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

FreeBSD implements dup3 like this in libc.

Change-Id: I36e37bc61c2e31561adb49001f287764125a74de
Reviewed-on: https://go-review.googlesource.com/c/go/+/355571
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>