enable appending to the remote file
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_APPEND, long append);
A parameter set to 1 tells the library to append to the remote file instead of
overwrite it. This is only useful when uploading to an FTP site.
This option was known as CURLOPT_FTPAPPEND up to 7.16.4
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.