make non-waiting paths of sem_[timed]wait and pthread_join cancelable

per POSIX these functions are both cancellation points, so they must
act on any cancellation request which is pending prior to the call.
previously, only the code path where actual waiting took place could
act on cancellation.

src/thread/pthread_join.c

@@ -8,6 +8,7 @@ static void dummy(void *p)
 int pthread_join(pthread_t t, void **res)
 {
 	int tmp;
+	pthread_testcancel();
 	while ((tmp = t->tid)) __timedwait(&t->tid, tmp, 0, 0, dummy, 0, 0);
 	if (res) *res = t->result;
 	if (t->map_base) munmap(t->map_base, t->map_size);

src/thread/sem_timedwait.c

@@ -8,6 +8,8 @@ static void cleanup(void *p)
 
 int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
 {
+	pthread_testcancel();
+
 	if (!sem_trywait(sem)) return 0;
 
 	int spins = 100;