os: dup pidfd if caller sets PidFD manually

os: dup pidfd if caller sets PidFD manually

Fixes #68984
Change-Id: I16d25777cb38a337cd4204a8147eaf866c3df9e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/607695
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>