os: avoid symlink races in RemoveAll on Windows

os: avoid symlink races in RemoveAll on Windows

Make the openat-using version of RemoveAll use the appropriate
Windows equivalent, via new portable (but internal) functions
added for os.Root.

We could reimplement everything in terms of os.Root,
but this is a bit simpler and keeps the existing code structure.

Fixes #52745

Change-Id: I0eba0286398b351f2ee9abaa60e1675173988787
Reviewed-on: https://go-review.googlesource.com/c/go/+/661575
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>