INT 21 - Phar Lap 386/DOS-Extender - MODIFY MS-DOS MEMORY BLOCK
        AX = 25C2h
        BX = new requested block size in paragraphs
        CX = real-mode paragraph address of memory block to modify
Return: CF clear if successful
            EAX destroyed
        CF set on error
            AX = error code
                07h MS-DOS memory control blocks destroyed
                08h insufficient memory
                09h invalid memory block address in CX
            BX = size in paragraphs of largest available memory block
SeeAlso: AX=25C1h