curl_slist_append - add a string to an slist（单链表）
struct curl_slist *headerlist=NULL;
static const char buf[] = "Expect:"
headerlist = curl_slist_append(headerlist, buf);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTPHEADER, struct curl_slist *headers);
官方说明：Pass a pointer to a linked list of HTTP headers to pass to the server and/or proxy in your HTTP request.
curl_formadd
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTPPOST, struct curl_httppost *formpost);
CURLOPT_HTTPPOST explained：CURLOPT_HTTPPOST - specify the multipart formpost content