implement panic database-save

[westram]

The main ARB-application (ie. every database-server) shall implement a panic-database-save:

• GBK_terminate shall allow to install callbacks:
  • each of these callbacks will be called once before ARB really terminates
  • if such a callback calls GBK_terminate itself, it should
    • inform about the failure and
    • continue with the next callback from the list or
    • finally really terminate.
• install a panic-save-callback (ARB_NTREE)

Possible behavior of panic-save-callback:

• free memory reserved for panic-save
• abort all running transactions
• prompt whether database shall be panic-saved?
  • save database (consider ascii-save into zip-stream)
    and/or
  • terminate