Introduce a dedicated networking.proxy option

Following the discussion NixOS#5021:
- obsolete the nix.proxy option
- add the networking.proxy option
- open a default no_proxy environment variable
- add a rsync option
- Manual tests ok.
- Automatic tests ok.

Amended by lethalman to simplify the option descriptions.
Ensure nix-daemon sees the proxy defined options

(Not sure about this one)

Here are the incomplete and a little contradictory discussions about
it (or at least my interpretations of them):

- https://github.com/NixOS/nixpkgs/pull/5058#discussion_r20830855
- https://github.com/NixOS/nixpkgs/pull/5058#discussion_r21043552

cc @edolstra @wmertens

What did I get wrong?
Improve readability - from https://github.com/NixOS/nixpkgs/pull/5058#discussion_r21043552
nixos/tests: add networking-proxy.nix to release
Merge branch 'create-networking-proxy-function' from PR #5058