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.
> Or even better, to have cfg.proxy.http-proxy have a default value of cfg.proxy.default.

Aha! I tried this but could not test at the time and forgot about it.
Will try again!

Thanks!

Ok!