INT 2F - AVATAR Serial Dispatcher - INSTALL IRQ3 HANDLER
        AX = 1A42h
        BX = 4156h ('AV')
        ES:DI -> FAR handler for serial port using IRQ3
        DS = data segment needed by handler
Return: AX = 1A42h if ASD not installed
           = 0000h if no more room
           else handle to use when uninstalling
Notes:  the handler need not save/restore registers or signal EOI to the
          interrupt controller
        the handler should return AX=0000h if the interrupt was meant for it,
          and either leave AX unchanged or return a non-zero value otherwise
        the most recently installed handler will be called first, continuing
          to earlier handlers until one returns AX=0000h
SeeAlso: AX=1A62h