Using "modular services"

"modular services" --- https://github.com/NixOS/nixpkgs/issues/428084 --- has finally landed. This is supposed to be very good for portability (other inits, others OSes, etc.). I have long hoped that would help deduplicate (and then upstream) NixBSD with NixOS. Now that it's merged upstream, it's a pretty good time to try it out!