Allow running NixOS services outside of systemd

The attribute ‘config.systemd.services.<service-name>.runner’
generates a script that runs the service outside of systemd.  This is
useful for testing, and also allows NixOS services to be used outside
of NixOS.  For instance, given a configuration file foo.nix:

  { config, pkgs, ... }:

  { services.postgresql.enable = true;
    services.postgresql.package = pkgs.postgresql92;
    services.postgresql.dataDir = "/tmp/postgres";
  }

you can build and run PostgreSQL as follows:

  $ nix-build -A config.systemd.services.postgresql.runner -I nixos-config=./foo.nix
  $ ./result

This will run the service's ExecStartPre, ExecStart, ExecStartPost and
ExecStopPost commands in an appropriate environment.  It doesn't work
well yet for "forking" services, since it can't track the main
process.  It also doesn't work for services that assume they're always
executed by root.