18.104.22.168: better end-of-the-world handling
* Implicit QUIT.
* Make sure we have %END-OF-THE-WORLD visible while doing cleanups.
* Handle errors from cleanups.
* Disable interrupts in places where the environment might now be up to
handling them yet/anymore.
* If user calls QUIT, and exit hooks run without serious trouble, report the
exit status requested even if there is trouble later flushing streams, etc.
* Also make SAVE-LISP-AND-DIE signal an error if SAVE returns. (When there is
eg. a file with the desired name but insufficient permissions.)