364b952d6 switched containerboot to partial netmap fetching, but
stopped refreshing `DNS.ExtraRecords`, so Tailscale Services created
after pod boot were invisible to resolveTailnetFQDN. To fix we watch
for SelfChange ipn bus notifies, and refetch dns-config via LocalAPI
to get a fresh set of `DNS.ExtraRecords`.
Fixes#20233
Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>