LWIP_ARP_FILTER_NETIF macro

Define this to 1 and define LWIP_ARP_FILTER_NETIF_FN(pbuf, netif, type) to a filter function that returns the correct netif when using multiple netifs on one hardware interface where the netif's low-level receive routine cannot decide for the correct netif (e.g. when mapping multiple IP addresses to one hardware interface).