daemon: Warn on old clients passing unexpected plugin-files.

The setting itself was already ignored due to exception trying to set pluginFiles.

src/libstore/daemon.cc

@@ -236,6 +236,10 @@ struct ClientSettings
  // the daemon, as that could cause some pretty weird stuff
  if (parseFeatures(tokenizeString<StringSet>(value)) != settings.experimentalFeatures.get())
  debug("Ignoring the client-specified experimental features");
+ } else if (name == settings.pluginFiles.name) {
+ if (tokenizeString<Paths>(value) != settings.pluginFiles.get())
+ warn("Ignoring the client-specified plugin-files.\n"
+ "The client specifying plugins to the daemon never made sense, and was removed in Nix >=2.14.");
  }
  else if (trusted
  || name == settings.buildTimeout.name