go.net/ipv6: make it work with Go 1.1

This CL makes use of built-in syscall stuff to allow go.net/ipv6
to work together with Go 1.1. Also it's able to improve the package
without churning the Go standard library.

Fixes golang/go#6548.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/15040044
17 files changed