Mark the event_err() functions as __attribute__((noreturn))

Mark the event_err() functions as __attribute__((noreturn))

This attribute tells gcc (and anything else that understands gcc
attributes) that the functions will never return control, and helps
the optimizer a little.  With luck, it will also tell
less-than-full-program dataflow analysis tools that they don't need to
worry about any code path that involves calling one of these functions
and then returning.

This patch also forces event_exit() to always exit, no matter what the
user-supplied fatal_callback does.  This means that the old unit tests
for the event_err* functions don't work any more, since they assume it
is safe to call event_err* if you've given it a bogus fatal_callback
that doesn't exit.  Instead, we have to make the unit tests fork
before calling event_err(), and have the main unit test process wait
for the event_err() test to exit with a sane exit code.  On unix,
that's trivial.  On windows, let's not bother and just assume that
event_err* works.