This change comes from CL 5536043,
created by Andrey Mirtchovski. His
description follows:

"The plan9 exec child handler does not manage
dup-ed fds from the parent correctly: when a
dup-ed file descriptor appears in the child's fd
list it is closed when first encountered and then
subsequent attempt to dup it later in Pass 2 fails,
resulting in 'fork/exec: fd out of range or not
open'."

R=golang-dev, rminnich, ality
CC=golang-dev, mirtchovski, rsc
https://golang.org/cl/6009046