Parameterise the path to nixpkgs.

This makes it possible to build several NixOS systems that use different
nixpkgs in the same nix-build invocation. Today, you can't do that since
the <nixpkgs> path reference is hard-coded in lib/eval-config.nix.
linuxPackages_2_6_32_xen is gone
NixOS channel: Use XZ compression for nixexprs.tar
Add an option ‘environment.interactiveShellInit’

This is like ‘environment.shellInit’, but only for interactive shells.
Provide missing command suggestions, Ubuntu style

If the user tries to run a program that doesn't exist from Bash, the
program name is looked up in a database that maps to Nix package
names.  If it is found, we print out a message like:

  $ pdflatex
  The program ‘pdflatex’ is currently not installed. It is provided by
  several packages. You can install it by typing one of the following:
    nix-env -i tetex
    nix-env -i texlive-core

If the environment variable $NIX_AUTO_INSTALL is set, the command is
installed and executed automatically:

  $ hello --version
  The program ‘hello’ is currently not installed. It is provided by
  the package ‘hello’, which I will now install for you.
  installing `hello-2.8'
  hello (GNU hello) 2.8
  Copyright (C) 2011 Free Software Foundation, Inc. ...

To use this, you must currently manually put the SQLite programs
database in /var/lib/nixos/programs.sqlite.  In the future, this file
should be provided as part of the NixOS channel so it gets updated
automatically.  To get a test version:

  $ curl http://nixos.org/~eelco/programs.sqlite.xz | xz -d > /var/lib/nixos/programs.sqlite
Make xz available by default

NixOS depends on xz anyway, so we may as well make it available.