OpenBSD support

[mrdomino]

It'd be great if I could use the current rust-and-capnproto flavor of passacre on OpenBSD, but I can't. The current proximate blocker is that an old version of nix is transitively included as a dependency somehow, but there's probably more problems hiding behind that.

(N.B. the pre-rust version of passacre works fine.)

[charmander]

If you use a minimal set of passacre features and don’t mind C-header-based configuration, https://github.com/charmander/cpassacre is compatible and should work on OpenBSD.