INT 21 - Phar Lap 386/DOS-Extender VMM - UNLOCK PAGES
        AX = 251Bh
        EDX = number of pages to unlock
        if BL = 00h
            ECX = linear address of first page to unlock
        if BL = 01h
            ES:ECX -> pointer to first page to unlock
Return: CF clear if successful
        CF set on error
            EAX = error code
                09h invalid address range
SeeAlso: AX=EB07h,INT 31/AX=0601h