ipv6: plumb in the standard library of Go 1.9 by using internal/socket package

This change uses the internal/socket package to ensure that the ipv6
package works with all supported versions of the Go standard library.

Fixes golang/go#19051.

Change-Id: I74911a8c5bba79e082a10d64e621ed415be1c033
Reviewed-on: https://go-review.googlesource.com/37042
Reviewed-by: Ian Lance Taylor <iant@golang.org>