* Give unpacked channels more sensible names than 0, 1, ... They now

  get the basename of the channel URL (e.g., nixpkgs-unstable).  The
  top-level Nix expression of the channel is now an attribute set, the
  attributes of which are the individual channels (e.g.,
  {nixpkgs_unstable = ...; strategoxt_unstable = ...}).  This makes
  attribute paths ("nix-env -qaA" and "nix-env -iA") more sensible,
  e.g., "nix-env -iA nixpkgs_unstable.subversion".