DragonFly support fix (for 5.8.x and forward).

The pthread slot approach is somewhat buggy (pretty visible
 with the stress unit test which segfault more or less randomly,
 but the stats never show up).
Using the default approach instead, the test passes eventough
 it s relatively slow (e.g 1.5 sec on FreeBSD vs 4.5 on DragonFly with same
 machine).

test/test-stress.c

@@ -306,7 +306,7 @@ static void run_os_threads(size_t nthreads, void (*fun)(intptr_t)) {
   pthread_t* threads = (pthread_t*)custom_calloc(nthreads,sizeof(pthread_t));
   memset(threads, 0, sizeof(pthread_t) * nthreads);
   //pthread_setconcurrency(nthreads);
-  for (uintptr_t i = 0; i < nthreads; i++) {
+  for (size_t i = 0; i < nthreads; i++) {
     pthread_create(&threads[i], NULL, &thread_entry, (void*)i);
   }
   for (size_t i = 0; i < nthreads; i++) {