Issue #12469: partial revert of 024827a9db64, freebsd6 thread initialization

* Don't create a thread at startup anymore to initialize the pthread library:
   it changes the behaviour of many functions related to signal handling like
   sigwait()
 * Reenable test_sigtimedwait_poll() on FreeBSD 6

Lib/test/test_signal.py

@@ -670,9 +670,6 @@ class PendingSignalsTests(unittest.TestCase):
 
     @unittest.skipUnless(hasattr(signal, 'sigtimedwait'),
                          'need signal.sigtimedwait()')
-    # issue #12303: sigtimedwait() takes 30 seconds on FreeBSD 6 (kernel bug)
-    @unittest.skipIf(sys.platform =='freebsd6',
-        "sigtimedwait() with a null timeout doens't work on FreeBSD 6")
     def test_sigtimedwait_poll(self):
         # check that polling with sigtimedwait works
         self.wait_helper(signal.SIGALRM, '''

Python/thread_pthread.h

@@ -144,10 +144,7 @@ typedef struct {
  * Initialization.
  */
 
-/* On FreeBSD6, pthread_kill() doesn't work on the main thread before
-   the creation of the first thread */
-#if defined(_HAVE_BSDI) \
-    || (defined(__FreeBSD__) && __FreeBSD_version < 700000)
+#if defined(_HAVE_BSDI)
 static
 void _noop(void)
 {