Allow building/testing individual systemd units
Fix broken Conflicts options
Apply better type checking to unitConfig/serviceConfig/...

In particular, complain if two modules define the same systemd option.
Use types.lines for script/preStart/postStart/postStop
Allow overrides to work for unit options
Check whether fileSystems defines the root FS
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.
httpd.nix: Support non-root operation
firefox: Update to 25.0.1
polkit: The rule file needs to end in .rules

Otherwise it's ignored.
polkit: Fix authenticating as a wheel user

In Javascript-based PolKit, "unix-user:0;unix-group:wheel" is not
valid; it should be a list "unix-user:0", "unix-group:wheel".
polkit: Add some examples