* faulthandler_user() displays the tracebacks of all threads even if it is
   unable to get the state of the current thread
 * test_faulthandler: only release the GIL in test_gil_released() check
 * create check_signum() subfunction

 * faulthandler_cancel_dump_tracebacks_later() is responsible to set running
   to zero (so we don't need the volatile keyword anymore)
 * release locks if PyThread_start_new_thread() fails

assert(thread.running == 0) was wrong in a corner case

Always release the cancel join.

Fix also another corner case: _PyFaulthandler_Fini() called after setting
running variable to zero, but before releasing the join lock.

If the thread releases the join lock before the cancel lock, the thread may
sometimes still be alive at cancel_dump_tracebacks_later() exit. So the cancel
lock may be destroyed while the thread is still alive, whereas the thread will
try to release the cancel lock, which just crash.

Another minor fix: the thread doesn't release the cancel lock if it didn't
acquire it.

The split probably won't happen for a while, but I might as well lay the
groundwork now since I'll be adding new test modules before too long.

initfsencoding() was not called yet: detect bootstrap (startup) issues earlier.

called yet: detect bootstrap (startup) issues earlier.

Stop if the stack overflow doesn't occur after allocating 100 MB on the stack.

 * dump_traceback_later() => dump_tracebacks_later()
 * cancel_dump_traceback_later() => cancel_dump_tracebacks_later()