sockopt callback

The sockopt callback is set with CURLOPT_SOCKOPTFUNCTION:

curl_easy_setopt(handle, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);

The sockopt_callback function must match this prototype:

int sockopt_callback(void *clientp,
                     curl_socket_t curlfd,
                     curlsocktype purpose);

This callback function gets called by libcurl when a new socket has
been created but before the connect call, to allow applications to change
specific socket options.

The clientp pointer points to the private data set with
CURLOPT_SOCKOPTDATA:

curl_easy_setopt(handle, CURLOPT_SOCKOPTDATA, custom_pointer);

This callback should return:

• CURL_SOCKOPT_OK on success
• CURL_SOCKOPT_ERROR to signal an unrecoverable error to libcurl
• CURL_SOCKOPT_ALREADY_CONNECTED to signal success but also that the socket is
  in fact already connected to the destination