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).

scripts/install-nix-from-closure.sh

@@ -0,0 +1,34 @@
+#! /bin/sh -e
+
+nix=@nix@
+regInfo=@regInfo@
+
+if ! $nix/bin/nix-store --load-db < $regInfo; then
+    echo "$0: unable to register valid paths"
+    exit 1
+fi
+
+. @nix@/etc/profile.d/nix.sh
+
+if ! $nix/bin/nix-env -i @nix@; then
+    echo "$0: unable to install Nix into your default profile"
+    exit 1
+fi
+
+# Add nix.sh to the shell's profile.d directory.
+p=$NIX_LINK/etc/profile.d/nix.sh
+
+if [ -w /etc/profile.d ]; then
+    ln -s $p /etc/profile.d/
+elif [ -w /usr/local/etc/profile.d ]; then
+    ln -s $p /usr/local/etc/profile.d/
+else
+    cat <<EOF
+Installation finished.  To ensure that the necessary environment
+variables are set, please add the line
+
+  source $p
+
+to your shell profile (e.g. ~/.profile).
+EOF
+fi