1.0.25.21: handling of potential corruptions

1.0.25.21: handling of potential corruptions

- add corruption_warning_and_maybe_lose that prints a warning and
  loses depending on lose_on_corruption_p (false by default)
- use corruption_warning_and_maybe_lose when the control stack is
  exhausted and on memory faults
- use corruption_warning_and_maybe_lose on the lisp handlers of
  SIGILL, SIGBUS and SIGEMT, as invoking them is surely not a good
  sign.
- add --lose-on-corruption as a runtime option
- add --disable-ldb as a runtime option
- update the man page and the user manual
- HEAP-EXHAUSTED fixes:
  - exit pseduo atomic properly and do pending interrupt if needed
  - signalling HEAP-EXHAUSTED in a WITHOUT-INTERRUPTS is dangerous
- use --lose-on-corruption in make-target*.sh

Also, block blockable signals on lose() to prevent other threads,
timers and such from interfering. If only all threads could be stopped
somehow.