nixos/tor: harden ProcSubset and ProtectProc

[ju1m]

Motivation for this change

ProcSubset and ProtectProc are new systemd options available. AFAICS such hardening is not causing trouble.

systemd-analyze security tor before:

✗ ProcSubset=                                                 Service has full access to non-process /proc files (/proc subset=)                                    0.1                                                                                                                                                                                                                                                           
→ Overall exposure level for tor.service: 1.3 OK 🙂

systemd-analyze security tor after:

✓ ProcSubset=                                                 Service has no access to non-process /proc files (/proc subset=)                                         
→ Overall exposure level for tor.service: 1.1 OK 🙂

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[stale]

I marked this as stale due to inactivity. → More info

[ju1m]

Still has to be merged. @SuperSandro2000 could you help please?