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_self.c

@@ -16,21 +16,14 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/calls/calls.h"
-#include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
 #include "libc/thread/tls.h"
 
+STATIC_YOINK("_pthread_main");
+
 /**
  * Returns current POSIX thread.
  */
 pthread_t pthread_self(void) {
   return __get_tls()->tib_pthread;
 }
-
-static struct PosixThread pthread_main;
-__attribute__((__constructor__)) static void pthread_self_init(void) {
-  pthread_main.tid = gettid();
-  pthread_main.status = kPosixThreadMain;
-  __get_tls()->tib_pthread = (pthread_t)&pthread_main;
-}