The net package uses various textual representations for network
identifiers and locators on the Internet protocol suite as API.
In fact, the representations are the composition of subset of multple
RFCs: RFC 3986, RFC 4007, RFC 4632, RFC 4291 and RFC 5952.

RFC 4007 describes guidelines for the use of textual representation of
IPv6 addressing/routing scope zone and doesn't prohibit the format for
implementation dependent purposes, as in, specifying a literal IPv6
address and its connected region of routing topology as application
user interface. However, a non-literal IPv6 address, for example, a
host name, with a zone enclosed in square brackets confuses us because
a zone is basically for non-global IPv6 addresses and a pair of square
brackets is used as a set of delimiters between a literal IPv6 address
and a service name or transport port number.

To mitigate such confusion, this change makes JoinHostPort not enclose
non-literal IPv6 addresses in square brackets and SplitHostPort accept
the form "host%zone:port" to recommend that anything enclosed in
square brackets should be a literal IPv6 address.

Before this change:
	JoinHostPort("name%zone", "80") = "[name%zone]:80"
	JoinHostPort("[::1%zone]", "80") = "[::1%zone]:80"
	SplitHostPort("name%zone:80") = "", "", "address name%zone:80: missing brackets in address"
	SplitHostPort("[name%zone]:80") = "name%zone", "80", nil
	SplitHostPort("[::1%zone]:80") = "::1%zone", "80", nil

After this change:
	JoinHostPort("name%zone", "80") = "name%zone:80"
	JoinHostPort("[::1%zone]", "80") = "[::1%zone]:80"
	SplitHostPort("name%zone:80") = "name%zone", "80", nil
	SplitHostPort("[name%zone]:80") = "name%zone", "80", nil // for backwards compatibility
	SplitHostPort("[::1%zone]:80") = "::1%zone", "80", nil

Also updates docs and test cases on SplitHostPort and JoinHostPort for
clarification.

Fixes #18059.
Fixes #18060.

Change-Id: I5c3ccce4fa0fbdd58f698fc280635ea4a14d2a37
Reviewed-on: https://go-review.googlesource.com/40510
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>