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/worker-protocol.hh

@@ -45,6 +45,7 @@ typedef enum {
     wopOptimiseStore = 34,
     wopVerifyStore = 35,
     wopBuildDerivation = 36,
+    wopAddSignatures = 37,
 } WorkerOp;