Subject: Semaphore LAB

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Chakrit has asked an excellent question:

   What are the semantics of SEM_SET when the value
   of the semaphore is negative at the time of the SEM_SET??
   (i.e. there exists a non-empty wait queue).

There are a number of ugly ways to handle this so we
will take the easy way out.

If the value is negative simply
     return(-EINVAL)
and don't change the value.

Do the same thing if the caller attempts to
set the semaphore to a negative value.