INT 21 - Phar Lap 386/DOS-Extender - GET SEGMENT ATTRIBUTES
        AX = 2515h
        BX = segment selector of descriptor in GDT or LDT
Return: CF clear if successful
            CL = access-rights byte for segment
            CH = use-type bit (USE16 or USE32)
        ECX<16-31> destroyed
        CF set on error
            EAX = error code
                09h invalid segment selector in BX