Implement Enhanced DAD algorithm for IPv6 described in
draft-ietf-6man-enhanced-dad-13.

This basically adds a random nonce option (RFC 3971) to NS messages
for DAD probe to detect a looped back packet. This looped back packet
prevented DAD on some pseudo-interfaces which aggregates multiple L2 links
such as lagg(4).

The length of the nonce is set to 6 bytes. This algorithm can be disabled by
setting net.inet6.ip6.dad_enhanced sysctl to 0 in a per-vnet basis.

Reported by: hiren
Reviewed by: ae
Differential Revision: https://reviews.freebsd.org/D1835