Make more improvements to threading support

- fix rare thread exit race condition on openbsd
- pthread_getattr_np() now supplies detached status
- child threads may now pthread_join() the main thread
- introduce sigandset(), sigorset(), and sigisemptyset()
- introduce pthread_cleanup_push() and pthread_cleanup_pop()

libc/thread/pthread_attr_setschedparam.c

@@ -43,6 +43,6 @@
 int pthread_attr_setschedparam(pthread_attr_t *attr,
                                const struct sched_param *param) {
   if (!param) return EINVAL;
-  attr->schedparam = param->sched_priority;
+  attr->__schedparam = param->sched_priority;
   return 0;
 }