core: support IPv6 duplicate address detection
NMDevice detects the DAD failures by watching the removal of tentative addresses (happens for DAD of addresses with valid lifetime, typically discovered addresses) or changes to addresses with dadfailed flag (permanent addresses, typically link-local and manually configured addresses). It retries creation of link-local addresses itself and lets RDisc know about the rest so that it can decide if it's rdisc-managed address and retry with a new address. Currently NMDevice doesn't do anything useful about link-local address DAD failures -- it just fails the link-local address addition instead of just timing out, which happened before. RDisc just logs a warning and removes the address from the list. However, with RFC7217 stable privacy addresses the use of a different address and thus a recovery from DAD failures would be possible.
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