Add support of HTTPS_PROXY/https_proxy environment variables. They
works similar to FTP_PROXY/ftp_proxy environment variables for FTP
scheme. That is, if scheme of request URL is "https" fetch(3) checks
if following environment variables are set in this order.
- HTTPS_PROXY
- https_proxy
- HTTP_PROXY
- http_proxy
And the value of first one set is used as setting of proxy for https.