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_getinheritsched.c

@@ -23,6 +23,6 @@
  */
 int pthread_attr_getinheritsched(const pthread_attr_t *attr,
                                  int *inheritsched) {
-  *inheritsched = attr->inheritsched;
+  *inheritsched = attr->__inheritsched;
   return 0;
 }