ARP has a queue of packets that should be sent after name resolution.

ARP has a queue of packets that should be sent after name resolution.
ND6 did only hold a single packet.  Unify the logic and add a mbuf
hold queue to struct llinfo_nd6.  This is MP safe and queue limits
are tracked with atomic operations.  New function if_mqoutput() has
common code for ARP and ND6.  ln_saddr6 holds the source address
of the requesting packet.  That is easier than fiddling with mbuf
queue in nd6_ns_output().
OK kn@