PlainAuth originally refused to send passwords to non-TLS servers
and was documented as such.

In 2013, issue #5184 was filed objecting to the TLS requirement,
despite the fact that it is spelled out clearly in RFC 4954.
The only possibly legitimate use case raised was using PLAIN auth
for connections to localhost, and the suggested fix was to let the
server decide: if it advertises that PLAIN auth is OK, believe it.
That approach was adopted in CL 8279043 and released in Go 1.1.

Unfortunately, this is exactly wrong. The whole point of the TLS
requirement is to make sure not to send the password to the wrong
server or to a man-in-the-middle. Instead of implementing this rule,
CL 8279043 blindly trusts the server, so that if a man-in-the-middle
says "it's OK, you can send me your password," PlainAuth does.
And the documentation was not updated to reflect any of this.

This CL restores the original TLS check, as required by RFC 4954
and as promised in the documentation for PlainAuth.
It then carves out a documented exception for connections made
to localhost (defined as "localhost", "127.0.0.1", or "::1").

Change-Id: I1d3729bbd33aa2f11a03f4c000e6bb473164957b
Reviewed-on: https://go-review.googlesource.com/68170
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>