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.
We generally use `null` rather than an empty string to denote an unset value. I.e.
```
type = types.nullOr types.str;
default = null;
```