Updated instructions and scripts for non-NixOS multi-user installations

To do a multi-user Nix installation on regular Linux distros, one detail is missing. The gcroots and profile directories need the right permissions. Without it per-user profiles cannot be created.

Furthermore, I've created a profile.d script containing all stuff needed for multi-user Nix installation (ported from NixOS), as the original nix.sh is missing quite a lot.

I've also created an init.d script for starting and stopping the nix-daemon.

More info about my findings: http://sandervanderburg.blogspot.com/2013/06/setting-up-multi-user-nix-installation.html

I don't know yet if nix.sh and nix-multiuser.sh should both be installed. Maybe we can implement a configure switch for this, or merge nix.sh with nix-multiuser.sh. I need some feedback on this.