crypto/tls: enforce that either ServerName or InsecureSkipVerify be given.

crypto/tls has two functions for creating a client connection: Dial,
which most users are expected to use, and Client, which is the
lower-level API.

Dial does what you expect: it gives you a secure connection to the host
that you specify and the majority of users of crypto/tls appear to work
fine with it.

Client gives more control but needs more care. Specifically, if it
wasn't given a server name in the tls.Config then it didn't check that
the server's certificates match any hostname - because it doesn't have
one to check against. It was assumed that users of the low-level API
call VerifyHostname on the certificate themselves if they didn't supply
a hostname.

A review of the uses of Client both within Google and in a couple of
external libraries has shown that nearly all of them got this wrong.

Thus, this change enforces that either a ServerName or
InsecureSkipVerify is given. This does not affect tls.Dial.

See discussion at https://groups.google.com/d/msg/golang-nuts/4vnt7NdLvVU/b1SJ4u0ikb0J.

Fixes issue 7342.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/67010043

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