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, 9 insertions, 0 deletions

diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 7893f2a4c..4d5d689dc 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -554,6 +554,15 @@ bool RemoteStore::verifyStore(bool checkContents, bool repair)
 }
 
 
+void RemoteStore::addSignatures(const Path & storePath, const StringSet & sigs)
+{
+    auto conn(connections->get());
+    conn->to << wopAddSignatures << storePath << sigs;
+    conn->processStderr();
+    readInt(conn->from);
+}
+
+
 RemoteStore::Connection::~Connection()
 {
     try {