systemd-nspawn can react to SIGTERM and send a shutdown signal to the container
init process. use that instead of going through dbus and machined to request
nspawn sending the signal, since during host shutdown machined or dbus may have
gone away by the point a container unit is stopped.
to solve the issue that a container that is still starting cannot be stopped
cleanly we must also handle this signal in containerInit/stage-2.
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