Lcg/proxy var fixes

- http_proxy now sets `ENV['HTTP_PROXY']` and `ENV['http_proxy']` and same for the rest of the settings
- the `ENV['HTTP_PROXY']` will not be overridden if it already exists (and same for all the rest)
- https_proxy and ftp_proxy can be set to an `http://` URL (fixes corruption of the resultant ENV var)