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.
This should be done in `networking.nix`, i.e.
```
environment.sessionVariables = config.networking.proxy.envVars;
```
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.