Refactor module to use subclasses like user module.

* Basically the moving parts from the original service module arranged in
  subclasses.
* General structure and helper methods comes from the user module.
* Less forgiving to unsupported platforms: it requires a subclass per platform.
  (This makes it easier to work on one platform without having to think about.
  what other platform might be affected in unexpected ways).
* Now has basic OpenBSD support.
* Solaris support needs to be added.

Thanks to @dhozac for general advice and Linux testing.
Thanks to @bcoca for clearing up some FreeBSD questions.