arp_responder: copy request's vlan tag onto the answer (v2)

I hope this is the right way of resubmitting #82.  (I verified the correctness of this vlan tagging using only my own arp responder code.)

I haven't known about arp_helper since it's not in betta.  Somehow it was a surprise to me that raiseEvent can collect feedbacks from the listeners.  I'd volunteer to rewrite arp_respoder to use it, but I'm too busy right now.