Bump amount of queued packets in for unresolved ARP/NDP entries.

Currently we default to keep only 1 packet per unresolved entry.
Ability to queue more than one packet was added 10 years ago, in r215207, though the default hasn't changed since then.

Things have changed since that time. Systems tend to initiate multiple connections at once for a variety of reasons.

For example, recent PR252278 describe happy-eyeball DNS behaviour sending multiple requests to the DNS server.

The primary reason for value determination is memory consumption: remote actors should not be able to easily exhaust local memory by sending packets to unresolved arp/ND entries.

So we should land something around D22447 before merging it back to stable.

Linux folks bumped the limit multiple times: 2011 2017.
TL;DR: they moved limit from 3 packets to 64kb, then to the variable depending on the amount of available system memory.

With all that in mind, I'm going to bump the current limit to 16 and work on implementing neigh limits.
I guess it's a bit late to convert the neigh limits from per-packet to per-queue to have it hit 13.0, but we certainly need to convert it at some point.