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This PR adds support for the systemd notify protocol. Several status messagess are provided. We should likely add a common notify/status message for graceful. Replaces #21140 Signed-off-by: Andrew Thornton <art27@cantab.net> --------- Signed-off-by: Andrew Thornton <art27@cantab.net> Co-authored-by: ltdk <usr@ltdk.xyz> Co-authored-by: Giteabot <teabot@gitea.io>