Enable cancellation point in sem_wait, it is required by POSIX.

For pshared semaphore, this commit still does not enable cancellation
point, I think there should be a pthread_enter_cancellation_point_np
for libc to implement a safe cancellation point.

lib/libc/gen/sem.c

@@ -254,7 +254,19 @@ sem_unlink(const char *name)
 	return (ksem_unlink(name));
 }
 
-int
+static void
+decrease_nwaiters(void *arg)
+{
+	sem_t *sem = (sem_t *)arg;
+
+	(*sem)->nwaiters--;
+	/*
+	 * this function is called from cancellation point,
+	 * the mutex should already be hold.
+	 */
+	_pthread_mutex_unlock(&(*sem)->lock);
+}
+
 sem_wait(sem_t *sem)
 {
 	int	retval;
@@ -266,11 +278,15 @@ sem_wait(sem_t *sem)
 		goto RETURN;
 	}
 
+	_pthread_testcancel();
+
 	_pthread_mutex_lock(&(*sem)->lock);
 
 	while ((*sem)->count == 0) {
 		(*sem)->nwaiters++;
-		_pthread_cond_wait(&(*sem)->gtzero, &(*sem)->lock);
+		_pthread_cleanup_push(decrease_nwaiters);
+		pthread_cond_wait(&(*sem)->gtzero, &(*sem)->lock);
+		pthread_cleanup_pop(0);
 		(*sem)->nwaiters--;
 	}
 	(*sem)->count--;