New packages: libgksu, gksu

Please, add `libgksu` and `gksu` to `nixpkgs`.

These derivations are based on gentoo and archlinux packages for `libgksu` and `gksu`.

GKSu is a library that provides a Gtk+ frontend to `su` and `sudo`. It supports login shells and preserving environment when acting as a `su` frontend. It is useful to menu items or other graphical programs that need to ask a user's password to run another program as another user.