The objective here is that services which depend on global config
(e.g. kernel config or busybox options or static paths in the
filesystem) now live under config.system.service, and are added
to that collection by the module that defines the necessary state.
This is a first step: the services will be configured by a typechecked
attr set instead of the arbitrary arguments that
pkgs.liminix.networking.pppoe accepts
