INT 21 - Novell NetWare - SYNCHRONIZATION SERVICES - WAIT ON SEMAPHORE
        AX = C502h
        CX:DX = semaphore handle
        BP = timeout limit in timer ticks (1/18 sec)
                0000h return immediately if semaphore already zero or negative
Return: AL = status
            00h successful
            FEh timeout
            FFh invalid handle
Desc:   decrement the semaphore's value, optionally waiting until its value
          becomes positive before decrementing
Note:   this function is supported by NetWare 4.6+, Advanced NetWare 1.0+, and
          Alloy NTNX
SeeAlso: AX=C501h,AX=C503h