overridden by the threads library to provide a userland version
of non-pshared semaphores and cancellation points. Also add
a sem_timedwait().
The libc version of semaphores always uses kernel semaphores
regardless of whether pshared is set or not. When threads are
not present, it is difficult to get sem_wait() or sem_timedwait()
to do the right thing (since pthread_cond_timedwait() and
pthread_cond_wait() are stubs in libc and just return immediately).
