This is a counterpart of https://go-review.googlesource.com/37039.

This change introduces a package that provides a portable interface
for the manipulation of sockets using either syscall.Conn and
syscall.RawConn interfaces or the internal/netreflect package
appropriately.

The package ensures that a package using this works with all supported
versions of the Go standard library.

Updates golang/go#19051.

Change-Id: Ib72ea369e6839e77fed6e35b9aedc364e73c51cb
Reviewed-on: https://go-review.googlesource.com/37035Reviewed-by: Ian Lance Taylor <iant@golang.org>