- change runtime_pollWait so it does not return
  closed or timeout if IO is ready - windows must
  know if IO has completed or not even after
  interruption;
- add (*pollDesc).Prepare(mode int) that can be
  used for both read and write, same for Wait;
- introduce runtime_pollWaitCanceled and expose
  it in net as (*pollDesc).WaitCanceled(mode int);

Full windows netpoll changes are
here https://golang.org/cl/8670044/.

R=golang-dev, dvyukov
CC=golang-dev
https://golang.org/cl/10485043