This new command will ask the journal daemon to flush all log data
stored in /run to /var, and wait for it to complete. This is useful, so
that in case of Storage=persistent we can order systemd-tmpfiles-setup
afterwards, to ensure any possibly newly created directory in /var/log
gets proper access mode and owners.
The old automatism that the flushing of the journal from /run to /var
was triggered by the appearance of /var/log/journal is broken if that
directory is mounted from another host and hence always available to be
useful as mount point. To avoid probelsm with this, introduce a new unit
that is explicitly orderer after all mounte files systems and triggers
the flushing.