Ensure we are in accept state in DTLSv1_listen

Calling SSL_set_accept_state() after DTLSv1_listen() clears the state, so
SSL_accept() no longer works. In 1.0.2 calling DTLSv1_listen() would set
the accept state automatically. We should still do that.

Fixes #1989

Reviewed-by: Andy Polyakov <appro@openssl.org>