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.
@edolstra

> BTW, I think the Nix daemon won't see the proxy variables now. You probably need to pass them via systemd.services.nix-daemon.environment as well. 

I suppose you say that because there is no relation between the nix-daemon and this new extracted option, am I on the right track?