Stop DTLS servers asking for unsafe legacy renegotiation

If a DTLS client that does not support secure renegotiation connects to an
OpenSSL DTLS server then, by default, renegotiation is disabled. If a
server application attempts to initiate a renegotiation then OpenSSL is
supposed to prevent this. However due to a discrepancy between the TLS and
DTLS code, the server sends a HelloRequest anyway in DTLS.

This is not a security concern because the handshake will still fail later
in the process when the client responds with a ClientHello.

Reviewed-by: Tim Hudson <tjh@openssl.org>