net/url: allow Parse, ParseRequestURI to parse ipv6 zone identifiers in URIs
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.