faulthandler.enable(all_threads=True) dumps the tracebacks even if it is not
possible to get the state of the current thread
Create also the get_thread_state() subfunction to factorize the code.
* Write a new test to ensure that dump_tracebacks_later() still works if
it was already called and then cancelled before
* Don't use a variable to check the status of the thread, only rely on locks
* The thread only releases cancel_event if it was able to acquire it (if
the timer was interrupted)
* The main thread always hold this lock. It is only released when
faulthandler_thread() is interrupted until this thread exits, or at Python
exit.
The thread must not receive any signal. If the thread receives a signal,
sem_timedwait() is interrupted and returns EINTR, but in this case,
PyThread_acquire_lock_timed() retries sem_timedwait() and the main thread is
not aware of the signal. The problem is that some tests expect that the main
thread receives the signal, not faulthandler handler, which should be
invisible.
On Linux, the signal looks to be received by the main thread, whereas on
FreeBSD, it can be any thread.
