[release-branch.go1.12] os: enable the close-on-exec flag for openFdAt
There's a race here with fork/exec, enable the close-on-exec flag
for the new file descriptor.
Updates #33405
Fixes #33424
Change-Id: Ib1e405c3b48b11c867f183fd13eff8b73d95e3b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/188537
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit
2d6ee6e89a4b30c7528d2977df4e1aa86651e4e4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/188538
Run-TryBot: Ian Lance Taylor <iant@golang.org>