internal/poll: be explicit when using runtime netpoller

internal/poll: be explicit when using runtime netpoller

internal/poll package assumes that only net sockets use runtime
netpoller on windows. We get memory corruption if other file
handles are passed into runtime poller. Make FD.Init receive
and use useNetpoller argument, so FD.Init caller is explicit
about using runtime netpoller.

Fixes #21172

Change-Id: I60e2bfedf9dda9b341eb7a3e5221035db29f5739
Reviewed-on: https://go-review.googlesource.com/65810
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>