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_getdetachstate.c

@@ -27,6 +27,6 @@
  * @return 0 on success, or error on failure
  */
 int pthread_attr_getdetachstate(const pthread_attr_t *attr, int *detachstate) {
-  *detachstate = attr->detachstate;
+  *detachstate = attr->__detachstate;
   return 0;
 }