INT 21 - Phar Lap 386/DOS-Extender - RELEASE MS-DOS MEMORY BLOCK
        AX = 25C1h
        CX = real-mode paragraph address of memory block to free
Return: CF clear if successful
            EAX destroyed
        CF set on error
            AX = error code
                07h MS-DOS memory control blocks destroyed
                09h invalid memory block address in CX
SeeAlso: AX=25C2h