- Add an AFS_PRE_SHUTDOWN hook called before shutting down the client.

- Add an AFS_PRE_SHUTDOWN hook called before shutting down the client.
    - Provide a shutdown hook, disabled by default, that kills all
      processes with files open in /afs.  Enabling this ensures a more
      reliable clean shutdown and umounting of the cache partition, but
      may not always be what the admin wants.

This duplicates the functionality in the upstream init script (where it
calls /usr/vice/etc/killafs if present), but in a more general way and
with better documentation.