ARP cache states

Also written: REACHABLE, STALE, DELAY, PROBE, INCOMPLETE, FAILED

The state `ip neigh` shows for each entry: REACHABLE (confirmed in about the last 30 seconds), STALE (not confirmed lately and checked on the next send), DELAY or PROBE (being checked now), INCOMPLETE (a request is out with no reply yet) and FAILED (nobody answered).