journald: downgrade various log messages from LOG_WARNING to LOG_INFO

journald: downgrade various log messages from LOG_WARNING to LOG_INFO

None of these conditions are real issues, but they can simply happen
because we just swtched from /run to /var as backend for logging and
there are old files from different boots with different systemd versions
and so on.

Let's not make more noise than necessary: still log, but not consider it
a warning, but just some normal thing.

We are handling these issues safely after all: by rotating and starting
anew, i.e. there's no reason to be concerned.