dhcp: wait DAD completion for DHCPv6 addresses

Wait that addresses received through DHCPv6 complete duplicate address
detection before reporting that the lease can be used.

Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')

(cherry picked from commit 393bc628)
59 jobs for bg/dhcp6-dad-on-1-38 in 34 minutes and 40 seconds (queued for 5 seconds)