Garbage collect unused links in /nix/store/.links

Incremental optimisation requires creating links in /nix/store/.links
to all files in the store.  However, this means that if we delete a
store path, no files are actually deleted because links in
/nix/store/.links still exists.  So we need to check /nix/store/.links
for files with a link count of 1 and delete them.

src/libstore/local-store.hh

@@ -264,6 +264,8 @@ private:
         
     int openGCLock(LockType lockType);
     
+    void removeUnusedLinks();
+
     void startSubstituter(const Path & substituter,
         RunningSubstituter & runningSubstituter);