INT 21 - Novell NetWare - SYNCHRONIZATION SERVICES - CLOSE SEMAPHORE
        AX = C504h
        CX:DX = semaphore handle
Return: AL = status
            00h successful
            FFh invalid handle
Desc:   decrement the semaphore's open count, and delete the semaphore if the
          count reaches zero
Note:   this function is supported by NetWare 4.6+, Advanced NetWare 1.0+, and
          Alloy NTNX
SeeAlso: AX=C501h