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_getguardsize.c

@@ -26,6 +26,6 @@
  */
 errno_t pthread_attr_getguardsize(const pthread_attr_t *attr,
                                   size_t *guardsize) {
-  *guardsize = attr->guardsize;
+  *guardsize = attr->__guardsize;
   return 0;
 }