Add a "root" parameter to local stores

This makes it easier to create a diverted store, i.e.

  NIX_REMOTE="local?root=/tmp/root"

instead of

  NIX_REMOTE="local?real=/tmp/root/nix/store&state=/tmp/root/nix/var/nix" NIX_LOG_DIR=/tmp/root/nix/var/log

src/libstore/store-api.hh

@@ -491,7 +491,9 @@ protected:
 class LocalFSStore : public Store
 {
 public:
+    const Path rootDir;
     const Path stateDir;
+    const Path logDir;
 
     LocalFSStore(const Params & params);