internal/poll: merge a couple of common definitions

(*FD).Shutdown and (*FD).RawControl were exactly identical in fd_unix.go
and fd_windows.go, so merge them into fd_posix.go.

Change-Id: Ie751edb06d293e08f1493682d6bc5f22e8f3e628
Reviewed-on: https://go-review.googlesource.com/c/go/+/224137


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>