INT 21 - Phar Lap 386/DOS-Extender - CHANGE SEGMENT ATTRIBUTES
        AX = 2514h
        BX = segment selector of descriptor in GDT or LDT
        CL = new access-rights byte
        CH = new use-type bit (USE16 or USE32)
Return: CF clear if successful
        CF set on error
            EAX = error code
                09h invalid selector in BX
SeeAlso: INT 31/AX=0009h