Change Post Handshake auth so that it is opt-in

Having post handshake auth automatically switched on breaks some
applications written for TLSv1.2. This changes things so that an explicit
function call is required for a client to indicate support for
post-handshake auth.

Fixes #6933.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6938)