[Fuzzer] Use user signal to coordinate handler shutdown (#82067)

This updates the signal handle thread coordinating to use a user signal
bit on the SignalHandlerEvent to coordinate shutdown instead of closing
the event handle. Closing the event handle is racy as the handle may be
closed before the signal handler thread resolves the handle value in
_zx_object_wait_many() and we would like to make this an explicit error.
Using the user signal bit 1 instead and then closing the event object
after the signal handler thread is joined cannot race as the wait will
terminate whether the signal is raised before or after the wait begins.