os,internal/poll: disassociate handle from IOCP in File.Fd

Go 1.25 will gain support for overlapped IO on handles passed to
os.NewFile thanks to CL 662236. It was previously not possible to add
an overlapped handle to the Go runtime's IO completion port (IOCP),
and now happens on the first call the an IO method.

This means that there is code that relies on the fact that File.Fd
returns a handle that can always be associated with a custom IOCP.
That wouldn't be the case anymore, as a handle can only be associated
with one IOCP at a time and it must be explicitly disassociated.

To fix this breaking change, File.Fd will disassociate the handle
from the Go runtime IOCP before returning it. It is then not necessary
to defer the association until the first IO method is called, which
was recently added in CL 661955 to support this same use case, but
in a more complex and unreliable way.

Updates #19098.

Cq-Include-Trybots: luci.golang.try:gotip-windows-amd64-race,gotip-windows-amd64-longtest,gotip-windows-arm64
Change-Id: Id8a7e04d35057047c61d1733bad5bf45494b2c28
Reviewed-on: https://go-review.googlesource.com/c/go/+/664455


Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>