Introduce new API for flags of the SSL bufferevent Introduce more generic API (like for evbuffer): - bufferevent_ssl_set_flags() - bufferevent_ssl_clear_flags() - bufferevent_ssl_get_flags() And deprecate existing: - bufferevent_openssl_get_allow_dirty_shutdown() - bufferevent_openssl_set_allow_dirty_shutdown() - bufferevent_mbedtls_get_allow_dirty_shutdown() - bufferevent_mbedtls_set_allow_dirty_shutdown()