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's more efficient to say:
```
http_proxy = if cfg.proxy.http-proxy != "" then cfg.proxy.http-proxy else cfg.proxy.default;
```
Or even better, to have `cfg.proxy.http-proxy` have a default value of `cfg.proxy.default`.