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
the Control Panel or Internet Explorer