title: curl options for libcurl method in download.file
body: <p>Is it possible to pass curl options when using <code>base::download.file</code> in R with <code>method = &quot;libcurl&quot;</code>?</p>
  <p>I need to pass those curl options in my environment:</p>
  <ul>
  <li>CURLOPT_PROXY (this is optional as the value for this is taken from the environment variables)</li>
  <li>CURLOPT_PROXYUSERNAME</li>
  <li>CURLOPT_PROXYAUTH</li>
  <li>CURLOPT_USERAGENT</li>
  </ul>
  <p>For example for a cmd it would be:</p>
  <pre><code>--proxy-negotiate --proxy-user ::: --user-agent &quot;Mozilla/5.0 (Windows NT 10.0) libcurl/7.64.1 r-curl/4.3.2 httr/1.4.4&quot;
  </code></pre>
  <p>I know there is <code>RCurl::getURL</code> and <code>curl::curl_download</code> where I can pass those curl options. I am bit surprised it is not possible with <code>base::download.file</code>.</p>