Custom pointer to the ftp chunk callbacks
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CHUNK_DATA, void *pointer);
Pass a pointer that will be untouched by libcurl and passed as the ptr argument to the CURL_CHUNK_BGN_FUNTION(3) and CURL_CHUNK_END_FUNTION(3).
Added in 7.21.0
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.