syscall: use fcntl F_DUP2FD_CLOEXEC in forkAndExecInChild on illumos

syscall: use fcntl F_DUP2FD_CLOEXEC in forkAndExecInChild on illumos

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

Illumos implements dup3 like this in libc.

Change-Id: I9782bce553ffb832e9b1a12bbf3c0a40c821f56e
Reviewed-on: https://go-review.googlesource.com/c/go/+/358374
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>