Using IPv6 link-local addresses to make connections between on-link
nodes is useful for small distributed applications but it requires zone
identifiers to distinguish a correct IP link. It's the same for
transports using URI for destination discovery such as HTTP, WebSocket.

This change allows Parse, ParseRequestURI functions and String method of
URL to parse/return a literal IPv6 address followed by a zone identifier
within a URI as described in RFC 6874.

Fixes #6530.

Change-Id: I2936ea65c1446994770cf2ee2c28a1c73faaa0ca
Reviewed-on: https://go-review.googlesource.com/2431Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>