Previously we would set SSL_OP_LEGACY_SERVER_CONNECT by default in
SSL_CTX_new(), to allow connections to legacy servers that did not
implement RFC 5746.

It has been more than a decade since RFC 5746 was published, so
there has been plenty of time for implmentation support to roll out.

Change the default behavior to be to require peers to support
secure renegotiation.  Existing applications that already cleared
SSL_OP_LEGACY_SERVER_CONNECT will see no behavior change, as
re-clearing the flag is just a little bit of redundant work.
The old behavior is still available by explicitly setting the flag
in the application.

Also remove SSL_OP_LEGACY_SERVER_CONNECT from SSL_OP_ALL, for
similar reasons.

Document the behavior change in CHANGES.md, and update the
SSL_CTX_set_options() and SSL_CONF_cmd manuals to reflect the change
in default behavior.

Fixes: 14848
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15127)