Add "nix copy-sigs" command

This imports signatures from one store into another. E.g.

  $ nix copy-sigs -r /run/current-system -s https://cache.nixos.org/
  imported 595 signatures

src/libstore/binary-cache-store.hh

@@ -170,6 +170,9 @@ public:
 
     ref<FSAccessor> getFSAccessor() override;
 
+    void addSignatures(const Path & storePath, const StringSet & sigs)
+    { notImpl(); }
+
 };
 
 }