INT 21 - Novell NetWare - SYNCHRONIZATION SERVICES - OPEN SEMAPHORE
        AX = C500h
        DS:DX -> semaphore name (counted string, max 127 bytes)
        CL = initial value for semaphore
Return: AL = status
            00h successful
                BL = number of processes having semaphore open
                CX:DX = semaphore handle
            FEh invalid name length
            FFh invalid semaphore value
Notes:  this function is supported by NetWare 4.6+, Advanced NetWare 1.0+, and
          Alloy NTNX
        the semaphore's value is incremented by AX=C503h and decremented by
          AX=C502h
SeeAlso: AX=C502h,AX=C503h,AX=C504h