Fix issues in IPv4 Duplicate Address Detection

Beniamino Galvani requested to merge bg/ipv4-dad-fixes into main


This branch addresses the failures found in the NMCI test suite when enabling IPv4 duplicate address detection (aka ACD, RFC 5227) by default.

The last commit is only to enable DAD by default during the CI pipelines and is not meant to be merged for now because the decision is not final yet.

Some of the NMCI failures are caused by bugs or race conditions in tests. Those are fixed by:

NetworkManager-ci!1498 (merged)

Supersedes MR !1730 (closed)

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