Implement process spawning on Windows

[Ericson2314]

#8901 just stubbed it out for now: we need to put an actual implementation in src/libutil/windows/processes.cc.

As always, a good resource is what was done in the original Windows port:

• src/libutil/util.cc

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-on-windows/1113/109

[roberth]

POSIX spawn was mentioned in another thread.
It'd be good to look into a generic process invoking interface that's implemented for either platform.

[Ericson2314]

Yes. The original port did indeed offer such an interface, and we should too.