* The determination of the root set should be made by the privileged

process, so forward the operation.
* Spam the user about GC misconfigurations (NIX-71).
* findRoots: skip all roots that are unreadable - the warnings with
  which we spam the user should be enough.

src/libstore/local-store.hh

@@ -64,6 +64,8 @@ public:
     void addIndirectRoot(const Path & path);
     
     void syncWithGC();
+
+    Roots findRoots();
 };