INT 21 - Phar Lap 386/DOS-Extender - SET INT TO ALWAYS GAIN CONTRL IN PROT MODE
        AX = 2506h
        CL = interrupt number
        DS:EDX = CS:EIP of protected-mode interrupt handler
Return: CF clear
Notes:  this function modifies both the real-mode low-memory interrupt
          vector table and the protected-mode Interrupt Descriptor Table (IDT)
        interrupts occurring in real mode are resignaled in protected mode
        this function is also supported by FlashTek X-32VM