journal: replace implicit flushing of journal by explicit one

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.