INT 21 - European MSDOS 4.0 - "SETPRI" - GET/SET PROCESS PRIORITY
        AX = 8E00h
        BH = 00h
        BL = action
            00h set priority for command subtree
            01h set priority for specified process only
        CX = Process ID
        DH = 00h
        DL = change in priority (00h to get priority)
Return: CF clear if successful
            DL = process priority
            DH destroyed
        CF set on error
            AX = error code (01h,no such process)(see AH=59h)
Desc:   specify or determine the execution priority of the specified process
          or the process and all of its children