EC2: Turn off IPv6 DAD

Disable Duplicate Address Detection in EC2 instances. The networking
configuration in EC2, with IPv6 addresses assigned by DHCPv6 and
host egress filtering, makes "duplicate addresses" impossible.

This speeds up the boot process in EC2 by 2 seconds.

Reviewed by: kp, imp, bz
MFC after: 2 weeks
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D33091