* server: save resume metadata before logind shutdown

Watch PrepareForShutdown with a delay inhibitor, write the existing
snapshot, then release the lock and stop. Startup does not wait on logind.

* server: keep test runs off the host shutdown lock

The harness sets SIGNALTTY_LOGIND=0 unless a private-bus test opts in.
Shutdown also stores a notify permit so a request between accepts is kept.

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Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>