runtime: change netpoll in preparation for windows implementation
- 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