bsd: init netbsd & openbsd userland

Adds a couple of useful NetBSD and OpenBSD derivations. Some of these
will be integrated into Nixpkgs later.

Noncomprehensive list:

- netbsd.getent
- netbsd.getconf
- netbsd.fts
- openbsd.mg
- netbsd.compat (can replace libbsd)