This change adds Zone field to IPNet structure for making it possible to
determine which network interface is associated with IPv6 link-local
address. Also makes ParseCIDR and IPNet.String capable handling literal
IPv6 address prefixes with zone identifier.

Fixes #14518.

Change-Id: I8f8a40d3b4f500ffef25728d4995651379d8408a
Reviewed-on: https://go-review.googlesource.com/19946Reviewed-by: Ian Lance Taylor <iant@golang.org>