LWIP_NETIF_HWADDRHINT macro

LWIP_NETIF_HWADDRHINT==1: Cache link-layer-address hints (e.g. table indices) in struct netif. TCP and UDP can make use of this to prevent scanning the ARP table for every sent packet. While this is faster for big ARP tables or many concurrent connections, it might be counterproductive if you have a tiny ARP table or if there never are concurrent connections.