Short description of concept of shutdown-thread

This is implemented, at least from side of the scripts.
Missing are handlers for epg-update from outside and autotimer-kicking.

* Thread before shutdown - With possibility to plug in some addons
  This thread will be started when
    + it is sure it can do the shutdown (nothing is against it)
    + and it is an automatic shutdown (perhaps changed in future)
    + and this thread has not been finished in the last XXmin

  When decided not to run this thread
    shutdown continues
  else
    shutdown is aborted (with normal retry mechanism).
    
  This thread can do various things
    + do an epg-scan svdrp-SCAN or switch channel-ranges per CHAN)
    + fetch epg from external providers tvmovie hoerzu (when online)
    + fetch epg from infosat or nxtvepg
    + go over all recordings and start noad for those which have not had it already
    + trigger the xxv oder vdradmin autotimer
   
  This thread runs under a name for which is explicitly checked when shutdown so that
  it will not shutdown when it is not finished.
  It writes the time it finished into a file (under /var/vdr/shutdown-data)

  When doing shutdown and the last finish-time read from file is not longer ago than XXmin
  then it will not be started again and shutdown continues.