client_hello_cb
c->client_hello_cb = cb;
SSL_client_hello_cb_fn client_hello_cb;
if (sctx->client_hello_cb != NULL) {
switch (sctx->client_hello_cb(ussl, &al, sctx->client_hello_cb_arg)) {
SSL_CTX_set_client_hello_cb(ctx, client_hello_cb, NULL);