Add "nix mount-store" command

This mounts an arbitrary Nix store on the specified mount point.

Typical usage:

  $ /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender --version
  bash: /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender: No such file or directory

  $ nix mount-store /tmp/mp --store https://cache.nixos.org?local-nar-cache=/tmp/nars

  $ unshare -m -r

  $ mount -o bind /tmp/mp /nix/store

  $ /nix/store/d0am5d8gwh2kfdcgyxh4y684mb5b2v54-blender-2.79/bin/blender --version
  [after a lot of downloading...]
  Blender 2.79 (sub 0)

One application is to replace the current remote store file access in
hydra-server implemented via "nix {cat,ls}-store", which doesn't work
all that well (e.g. it doesn't resolve symlinks properly).

Another application would be on-demand fetching of build inputs on
Hydra build slaves (to speed up builds that don't access their entire
closure). However, that will require a lot more machinery.

shell.nix

@@ -27,7 +27,7 @@ with import ./release-common.nix { inherit pkgs; };
       git
       mercurial
     ]
-    ++ lib.optional stdenv.isLinux libseccomp;
+    ++ lib.optionals stdenv.isLinux [ libseccomp fuse ];
 
   inherit configureFlags;