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_getschedparam.c

@@ -23,6 +23,6 @@
  */
 int pthread_attr_getschedparam(const pthread_attr_t *attr,
                                struct sched_param *param) {
-  *param = (struct sched_param){attr->schedparam};
+  *param = (struct sched_param){attr->__schedparam};
   return 0;
 }