journal: don't emit space usage message when opening the journal (#4190)

journal: don't emit space usage message when opening the journal (#4190)

This patch makes system_journal_open() stop emitting the space usage
message. The caller is now free to emit this message when appropriate.

When restarting the journal, we can now emit the message *after*
flushing the journal (if required) so that all flushed log entries are
written in the persistent journal *before* the status message.

This is required since the status message is always younger than the
flushed entries.

Fixes #4190.