Backport DHCPv6-DAD patches to 1.36
On 1.36 branch NM doesn't wait for IPv6 duplicate address detection to complete before signaling that the connection is ready. This is a regression from 1.34.
This MR backports 2fd5fb14 dhcp: wait DAD completion for DHCPv6 addresses
and c36bc32d dhcp: decline IPv6 lease if all adresses fail DAD
from nm-1-38, plus all the ~30 commits necessary to avoid conflicts. All commits applied cleanly.