dump_traceback_later() and cancel_dump_tracebacks_later() to
cancel_dump_traceback_later().

Call the previous signal handler if chain is True.

 * Set the default value of all_threads arguments to True
 * Py_FatalError() dumps all threads, instead of only the current thread

Dump only the current thread is not reliable. In some cases, Python is unable
to retrieve the state of the current thread and so is unable to dump the
traceback. faulthandler keeps a reference to the interpreter and so is always
able to dump the traceback of all threads.

Fix a crash: don't read from NULL.

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