journalctl: add new --flush command and make use of it in systemd-journal-flush.service

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.