INT 2F - AVATAR Serial Dispatcher - INSTALL IRQ4 HANDLER
        AX = 1A43h
        BX = 4156h ('AV')
        ES:DI -> FAR handler for serial port using IRQ4
        DS = data segment needed by handler
Return: AX = 1A43h if ASD not installed
           = 0000h if no more room
           else handle to use when uninstalling
Notes:  (see AX=1A42h)
SeeAlso: AX=1A63h