Revert garp to old system

[PlagueCZ]

This is a fix of a fix from #769 which introduced a bug in ARP which degraded performace. Network wos working but with occasional packet loss, that is why previous tests missed it.

I simply fully reverted the garp implementation to before #714, the rest of #769, i.e. DHCP and ND is kept.

Fixes #768

[byteocean]

if we want to update MAC address cache even if VM suddenly changes it during running, we can also do

if (sender_ip == htonl(port->iface.cfg.own_ip)) {
      dp_l2_addr_set(port, &incoming_eth_hdr->src_addr);
      port->iface.arp_done = true;  // stop further probing
      // fall through — don't consume
  }

But they are different approaches with very minor difference, so the change in this PR should be sufficient now.