dired-du-duc

This adapts Dired-Du to use "duc".  Get duc:

  https://duc.zevv.nl/
  https://github.com/zevv/duc

Enable with

  (global-dired-du-duc-mode)

You'll also want to remove any call to `dired-du-mode' in your initfiles.

The global mode does three things:

1. Asynchronously run "duc index" each time a Dired buffer is opened.
   Option `dired-du-duc-index-predicate' controls this; the default is to
   avoid doing this for remote network directories.

2. Turn `dired-du-duc-mode' on in relevant buffers when duc is ready.
   Option `dired-du-duc-mode-predicate' can be configured to enable it
   always, if you are fine with the slow "du" as a fallback.

3. Regularly re-index the directories we have previously indexed.
   Option `dired-du-duc-delay' controls how often to do this.