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

1 files changed, 12 insertions, 0 deletions

diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index 527f6bca9..439cd3dc0 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -543,6 +543,18 @@ static void performOp(ref<LocalStore> store, bool trusted, unsigned int clientVe
         break;
     }
 
+    case wopAddSignatures: {
+        Path path = readStorePath(from);
+        StringSet sigs = readStrings<StringSet>(from);
+        startWork();
+        if (!trusted)
+            throw Error("you are not privileged to add signatures");
+        store->addSignatures(path, sigs);
+        stopWork();
+        to << 1;
+        break;
+    }
+
     default:
         throw Error(format("invalid operation %1%") % op);
     }