This will make it possible to use the poller with the os package.

This is a lot of code movement but the behavior is intended to be
unchanged.

Update #6817.
Update #7903.
Update #15021.
Update #18507.

Change-Id: I1413685928017c32df5654ded73a2643820977ae
Reviewed-on: https://go-review.googlesource.com/36799
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>