internal/poll: add RawControl, RawRead and RawWrite methods to FD

This change adds RawControl, RawRead and RawWrite methods to type FD
to make the runtime-integrated network poller work together with a
user-defined function. The methods are used via the net package from
external packages and type FD is considered as an implementation of
syscall.Conn and syscall.RawConn interfaces.

Updates #19435.

Change-Id: I4ad04b10ffddb2b54fa8d70587440960d73c0a2d
Reviewed-on: https://go-review.googlesource.com/37038


Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>