Custom pointer to pass to sockopt callback
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SOCKOPTDATA, void *pointer);
Pass a pointer that will be untouched by libcurl and passed as the first argument in the sockopt callback set with CURLOPT_SOCKOPTFUNCTION(3).
The default value of this parameter is NULL.
Added in 7.16.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.