Block until IPv6 DAD completion

[Kaarle Ritvanen]

It would be convenient if ifup blocked until the IPv6 duplicate address detection completes for all active interfaces. Then it would be safe to start daemons binding to IPv6 addresses without additional checks.

Alpine Linux currently uses this hook script to achieve the same:
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/main/busybox/dad.if-up