nixos systemPackages: rework default outputs

- Now `pkg.outputUnspecified = true` but this attribute is missing in
  every output, so we can recognize whether the user chose or not.
  If (s)he didn't choose, we put `pkg.bin or pkg.out or pkg` into
  `systemPackages`.
- `outputsToLink` is replaced by `extraOutputsToLink`.
  We add extra outputs *regardless* of whether the user chose anything.
  It's mainly meant for outputs with docs and debug symbols.
- Note that as a result, some libraries will disappear from system path.

nixos/modules/security/polkit.nix

@@ -59,7 +59,7 @@ in
 
   config = mkIf cfg.enable {
 
-    environment.systemPackages = [ pkgs.polkit ];
+    environment.systemPackages = [ pkgs.polkit.bin pkgs.polkit.out ];
 
     systemd.packages = [ pkgs.polkit.out ];