Hi,

This patch makes qemu handle signals better.  For normal signals
(SIGINT, SIGTERM & friends) it just sets the request_shutdown flag,
making the main_loop exit and qemu taking the usual exit route, with
atexit handlers being called and so on, instead of qemu just being
killed by the signal.

For fatal signals such as SIGSEGV, where the exit handler can't return,
it allows to register clean up handlers which will be called instead of
the atexit handlers.  Those handlers must be save to run in signal context.

please apply,
  Gerd

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