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.
@wmertens Rereading this https://github.com/NixOS/nixpkgs/pull/5058#discussion_r20830855 comment, I saw that I forgot to address @edolstra's concern about nix-daemon.

This last commit tries to solve it according to my interpretation of @edolstra's suggestion.

I am not sure about this.
There is no test on nix-daemon so I do not know how to be certain of this.

If someone could reassure this is ok or propose something better, this would be great.

Cheers,