Add NetBSD/FreeBSD Support

Howdy! I'd like to request nix support for NetBSD and/or FreeBSD. There's a [wip port](https://wip.pkgsrc.org/cgi-bin/gitweb.cgi?p=pkgsrc-wip.git;a=tree;f=nix;h=bd6eebe083a9ebc7fa6f13083dd11623cb080fb1;hb=HEAD) but it fails to compile on NetBSD 