nix run: Mount the Nix store in a private namespace

This is a convenience command to allow users who are not privileged to
create /nix/store to use Nix with regular binary caches. For example,

  $ NIX_REMOTE="local?state=$HOME/nix/var&real=/$HOME/nix/store" nix run firefox bashInteractive

will download Firefox and bash from cache.nixos.org, then start a
shell in which $HOME/nix/store is mounted on /nix/store.

src/libstore/local-store.hh

@@ -73,6 +73,8 @@ private:
 
     Sync<State, std::recursive_mutex> _state;
 
+public:
+
     const Path realStoreDir;
     const Path dbDir;
     const Path linksDir;
@@ -80,6 +82,8 @@ private:
     const Path schemaPath;
     const Path trashDir;
 
+private:
+
     bool requireSigs;
 
     PublicKeys publicKeys;