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_setscope.c

@@ -19,6 +19,6 @@
 #include "libc/thread/thread.h"
 
 int pthread_attr_setscope(pthread_attr_t *a, int x) {
-  a->scope = x;
+  a->__scope = x;
   return 0;
 }