~/.cabal/config does not allow setting a http proxy

Cabal uses Network.HTTP.Proxy which looks up proxy settings via the `http_proxy` and `HTTP_PROXY` environment variables.

(Under Windows it also queries the registry for a proxy setting).

Thus, to use cabal behind a proxy one has to put all proxy information into an environment variable.

This is a security related issue when following conditions apply:
1. working on a multi-user system
2. a user is able to inspect the environment variables of other users (e.g. on Solaris this is the case)
3. Proxy access is restricted via username/password

Sure, one can argue that the issue is not that big because most users don't work on multi-user systems, even less use systems like Solaris and most proxy password are not that secret.

But still.

To fix this there are a few possibilities:
- add http proxy server/username/password options to ~/.cabal/config
- modify Network.HTTP.Proxy such that it also looks up possibly configured proxy settings via ~/.curlrc, ~/.wgetrc etc.

Start by reading Network.HTTP.Proxy and Cabal's ~/.cabal/config handling, then compare the current environment-variable and Windows registry lookup described in the issue. Decide which configuration source and proxy fields are in scope; done means Cabal can obtain proxy server and restricted credentials from user configuration without requiring environment variables.

Written by the indexing model from the issue text.