nix-store -q --roots / --gc --print-roots: Print temporary / in-memory roots

For example,

  $ nix-store -q --roots /nix/store/7phd2sav7068nivgvmj2vpm3v47fd27l-patchelf-0.8pre845_0315148
  {temp:1}

denotes that the path is only being kept alive by a temporary root
(i.e. /nix/var/nix/temproots/). Similarly,

  $ nix-store --gc --print-roots
  ...
  {memory:9} -> /nix/store/094gpjn9f15ip17wzxhma4r51nvsj17p-curl-7.53.1

shows that curl is being used by some process.

src/libstore/local-store.hh

@@ -176,7 +176,7 @@ private:
     typedef std::shared_ptr<AutoCloseFD> FDPtr;
     typedef list<FDPtr> FDs;
 
-    void readTempRoots(PathSet & tempRoots, FDs & fds);
+    PathSet readTempRoots(FDs & fds);
 
 public:
 
@@ -261,7 +261,9 @@ private:
 
     void findRoots(const Path & path, unsigned char type, Roots & roots);
 
-    void findRuntimeRoots(PathSet & roots);
+    Roots findRootsNoTemp();
+
+    PathSet findRuntimeRoots();
 
     void removeUnusedLinks(const GCState & state);