Support NetBSD in addition to Linux

[Ericson2314]

We can build a NetBSD kernel in Nixpkgs (or again, I'm not sure how well things worked for @matthewbauer originally). It's netbsd.sys.

A long time from now, I hope we can do #26850 i.e. NixOS/knetbsd and friends, but that's a really large project to start on, and I know very little about the latter stages of conventional OS startup bootstraping. I think I should start with this because it's much simpler.

CC @alyssais.