Create a networking.proxy option

Following the discussion from #5021

Code:
- obsolete the nix.proxy option
- add the networking.proxy option
- open a default no_proxy environment variable
- add a rsync option

Tests:
- Manual tests ok.
- Automatic tests added and ok

Note:
Code rebased to the latest code.
I do not know if it's a good idea.
It was at one point.
We cannot see this because I squashed the commit...
But here is the comment https://github.com/NixOS/nixpkgs/pull/5058#discussion_r20830855 which changed it.