new sysctl parameter:

net.ipv4.neigh.<interface name>.arp_antidote

that defines kernel behaviour when changes in correspondence between MAC
and IP are detected.

Parameter value 0 corresponds standart behaviour, ARP cache will be
silently updated.

Value=1..3 corresponds "verification" behaviour. Kernel will send ARP
request to test if there is a host at "old" MAC address. If such
response received it lets us know than one IP pretends to have
several MAC addresses at one moment, that probably caused by ARP spoof
attack.

Value=1 - just report attack and ignore spoofing attempt.

Value=2 - ARP cache record will be marked as "static" to prevent attacks
          in future.

Value=3 - ARP cache record will be marked as "banned", no data will be
          delivered to attacked IP anymore, untill system administrator
          unban ARP record updating it manually.
