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).
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.
Only warnings that could be fixed without changing the generated object
code and without restructuring the source code have been handled.
Reviewed by: /sbin/md5
so that multiple opens of the same semaphore without an intervening
sem_close() return the same object, and so that sem_close() does not
segfault while trying to remove the item from the list.
