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_setstacksize.c

@@ -28,6 +28,6 @@
  */
 errno_t pthread_attr_setstacksize(pthread_attr_t *a, size_t stacksize) {
   if (stacksize < PTHREAD_STACK_MIN) return EINVAL;
-  a->stacksize = stacksize;
+  a->__stacksize = stacksize;
   return 0;
 }