92f69f1544
Make the systemd-specific configuration of `ghostunnel`s service (see portability](https://github.com/NixOS/nixpkgs/blob/master/nixos/doc/manual/development/modular-services.md#portability-modular-service-portability). The idea here is that making the module easier to use in systems not using systemd could make for a best practice in writing such modules. As `ghostunnel` is the first such portable module, it seems all the more relevant to apply best practices there for when it is used as a source of inspiration in portable modules for further applications.