CURLOPT_SSL_CTX_FUNCTION to set a callback that gets called with the
   OpenSSL's ssl_ctx pointer passed in and allow a callback to act on it. If
   anything but CURLE_OK is returned, that will also be returned by libcurl
   all the way back. If this function changes the CURLOPT_URL, libcurl will
   detect this and instead go use the new URL.

   CURLOPT_SSL_CTX_DATA is a pointer you set to get passed to the callback set
   with CURLOPT_SSL_CTX_FUNCTION.