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/store-api.hh

@@ -346,6 +346,10 @@ public:
     /* Return an object to access files in the Nix store. */
     virtual ref<FSAccessor> getFSAccessor() = 0;
 
+    /* Add signatures to the specified store path. The signatures are
+       not verified. */
+    virtual void addSignatures(const Path & storePath, const StringSet & sigs) = 0;
+
     /* Utility functions. */
 
     /* Read a derivation, after ensuring its existence through