This change adds the syscall.Conn interface to Listener types, with the caveat that only RawConn.Control is supported. Custom socket options can now be set safely.

Updates #19435
Fixes #22065

Change-Id: I7e74780d00318dc54a923d1c628a18a36009acab
Reviewed-on: https://go-review.googlesource.com/71651
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>