nix-store -q --roots: Respect the gc-keep-outputs/gc-keep-derivations settings

So if a path is not garbage solely because it's reachable from a root
due to the gc-keep-outputs or gc-keep-derivations settings, ‘nix-store
-q --roots’ now shows that root.

src/libstore/remote-store.hh

@@ -40,6 +40,8 @@ public:
 
     Path queryDeriver(const Path & path);
     
+    PathSet queryValidDerivers(const Path & path);
+
     PathSet queryDerivationOutputs(const Path & path);
     
     StringSet queryDerivationOutputNames(const Path & path);