Pull up following revision(s) (requested by ozaki-r in ticket #1396):

sys/netinet6/nd6.h: revision 1.88
sys/netinet6/nd6_nbr.c: revision 1.174
sys/netinet6/nd6.c: revision 1.264
sys/netinet/if_arp.c: revision 1.288 (patch)

Initialize DAD components properly

The original code initialized each component in non-init functions such as
arp_dad_start and nd6_dad_find, conditionally based on a global flag for each.
However, it was racy because the flag and the code around it were not
protected by a lock and could cause a kernel panic at worst.

Fix the issue by initializing the components in bootup as usual.

(martin)