The IPv4 ARP cache used to indefinitely buffer packets in the Arpent hold list.
This is bad in case of a router, because it opens a 1 second
(retransmit time) window to leak all the to be forwarded packets.
This change makes the ipv4 arp code path similar to the IPv6 neighbour
solicitation path, using the retransmit process to time out old entries
(after 3 arp retransmits => 3 seconds).
A new function arpcontinue() has been added that unifies the point when
we schedule the (ipv6 sol retransmit) / (ipv4 arp timeout) and reduce
the hold queue to the last packet and unlock the cache.
As a bonus, we also now send a icmp host unreachable notification
for the dropped packets.
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