Modify the implementation of libc semaphores so that they can be overridden by the threads library to provide a userland version of non-pshared semaphores and cancellation points.  Also add a sem_timedwait().
Modify the implementation of libc semaphores so that they can be
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).

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