Bump amount of queued packets in for unresolved ARP/NDP entries to 16.

Currently default behaviour is 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 value was kep intact.

Things have changed since that time. Systems tend to initiate multiple
connections at once for a variety of reasons.
For example, recent kern/252278 bug report describe happy-eyeball DNS
behaviour sending multiple requests to the DNS server.

The primary driver for upper value for the queue length determination is
memory consumption. Remote actors should not be able to easily exhaust
local memory by sending packets to unresolved arp/ND entries.

For now, bump value to 16 packets, to match Darwin implementation.

The proper approach would be to switch the limit to calculate memory
consumption instead of packet count and limit based on memory.

We should MFC this with a variation of D22447.

Reviewers: manpages, network, bz, emaste

Reviewed By: emaste, gbe(doc), jilles(doc)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D28068