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/stdio/ftrylockfile.c

@@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/stdio/lock.internal.h"
 #include "libc/stdio/stdio.h"
+#include "libc/thread/thread.h"
 
 /**
  * Tries to acquire reentrant stdio object lock.