Use system proxies

Detected by checking the environment variables http_proxy, https_proxy, their uppercase variants and then all_proxy. Uses no_proxy environment variable as comma-separated list.

On Windows (and if previous detection fails), the registry is queried for `HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Internet Settings`. This is what you edit when you open "Internet Options" > "Connections" from he Control Panel or Internet Explorer