INT 21 - Phar Lap 386/DOS-Extender - SET REAL- & PROTECTED-MODE INT VECTORS
        AX = 2507h
        CL = interrupt numbern
        DS:EDX = CS:EIP of protected-mode interrupt handler
        EBX = CS:IP of real-mode interrupt handler
Return: CF clear
Notes:  interrupts are disabled until both vectors have been modified
        this function is also supported by FlashTek X-32VM
SeeAlso: AX=2505h