arp_helper: API-breaking improvements
arp_helper: API-breaking improvements

This breaks existing API/code, but hopefully the changes are all
big improvements.  Also, it should be better documented now.

MurphyMc/pox,noxrepo/pox,noxrepo/pox,MurphyMc/pox,noxrepo/pox,noxrepo/pox,MurphyMc/pox,MurphyMc/pox,MurphyMc/pox