Generate binary tarballs for installing Nix

For several platforms we don't currently have "native" Nix packages
(e.g. Mac OS X and FreeBSD).  This provides the next best thing: a
tarball containing the closure of Nix, plus a simple script
"nix-finish-install" that initialises the Nix database, registers the
paths in the closure as valid, and runs "nix-env -i /path/to/nix" to
initialise the user profile.

The tarball must be unpacked in the root directory.  It creates
/nix/store/... and /usr/bin/nix-finish-install.  Typical installation
is as follows:

  $ cd /
  $ tar xvf /path/to/nix-1.1pre1234_abcdef-x86_64-linux.tar.bz2
  $ nix-finish-install
  (if necessary add ~/.nix-profile/etc/profile.d/nix.sh to the shell
  login scripts)

After this, /usr/bin/nix-finish-install can be deleted, if desired.

The downside to the binary tarball is that it's pretty big (~55 MiB
for x86_64-linux).