npn_select_cb
ctx->ext.npn_select_cb = cb;
SSL_CTX_npn_select_cb_func npn_select_cb;
if (sctx->ext.npn_select_cb == NULL) {
if (sctx->ext.npn_select_cb(SSL_CONNECTION_GET_USER_SSL(s),
if (SSL_CONNECTION_GET_CTX(s)->ext.npn_select_cb == NULL
SSL_CTX_set_next_proto_select_cb(cctx, npn_select_cb, &idx);