Mark content-addressed paths in the Nix database and in .narinfo

This allows such paths to be imported without signatures.

src/nix/path-info.cc

@@ -73,6 +73,7 @@ struct CmdPathInfo : StorePathsCommand
                 std::cout << '\t';
                 Strings ss;
                 if (info->ultimate) ss.push_back("ultimate");
+                if (info->ca != "") ss.push_back("ca:" + info->ca);
                 for (auto & sig : info->sigs) ss.push_back(sig);
                 std::cout << concatStringsSep(" ", ss);
             }