os: separate Process.handle into a separate memory allocation
This is a step toward using AddCleanup rather than SetFinalizer
to close process handles.
For #70907
Change-Id: I7fb37461dd67b27135eab46fbdae94f0058ace85
Reviewed-on: https://go-review.googlesource.com/c/go/+/638575
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com>