net: make {TCP,UDP,IP,Unix}Conn compliant of syscall.Conn interface

This change makes {TCP,UDP,IP,Unix}Conn types compliant of
syscall.Conn interface and adds type rawConn as an implementation of
syscall.RawConn interface.

By this change, the long-standing issues regarding unsupported socket
options and system calls can be solved partly and the broken x/net
packages due to https://go-review.googlesource.com/36799 can be
repaired.

Fixes #3661.
Updates #9661.
Updates #19051.
Updates #19435.

Change-Id: Ic996b040418b54f6d043bc70591789d5a5b23270
Reviewed-on: https://go-review.googlesource.com/37039


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