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/local-store.hh

@@ -182,6 +182,8 @@ public:
 
     void setSubstituterEnv();
 
+    void addSignatures(const Path & storePath, const StringSet & sigs) override;
+
 private:
 
     Path schemaPath;