ARP has a sysctl to show the number of packets waiting for an arp

ARP has a sysctl to show the number of packets waiting for an arp
response.  Implement analog sysctl net.inet6.icmp6.nd6_queued for
ND6 to reduce places where mbufs can hide within the kernel.
Atomic operations operate on unsigned int.  Make the type of total
hold queue length consistent.
Use atomic load to read the value for the sysctl.  This clarifies
why no lock around sysctl_rdint() is needed.
OK mvs@ kn@