syscall: add Conn and RawConn interfaces

This change adds Conn and RawConn interfaces which can be used to
manipulate raw network connection end points typically represented as
socket descriptors.

Fixes #19435.

Change-Id: Ide2d28eeab91bfd27473ab47a87bec69950b64c9
Reviewed-on: https://go-review.googlesource.com/37913


Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>