os/user: support calling Current on impersonated threads
The syscall.OpenCurrentProcessToken call in user.Current fails
when called from an impersonated thread, as the process token is
normally in that case.
This change ensures that the current thread is not impersonated
when calling OpenCurrentProcessToken, and then restores the
impersonation state, if any.
Fixes #68647
Change-Id: I3197535dd8355d21029a42f7aa3936d8fb021202
Reviewed-on: https://go-review.googlesource.com/c/go/+/602415 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>