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.
> One nitpick, shouldn't this be cfg.proxy.envVars for sameness? 

I suddenly realise what you meant at first, 

Replace 
``` nix
environment.sessionVariables = config.networking.proxy.envVars;
```

by

``` nix
environment.sessionVariables = cfg.proxy.envVars;
```

am I right?