Make journald persistent and log to /var/log/journal

The lack of persistence in journald would break promtail, as /var/log/journal did not exist. Journald would then start logging to tempfiles in /var/run/journal, which would then get wiped each reboot. This is generally undesirable behaviour and therefore should be avoided.

This bugfix changes journald to always log to /var/log/journal, and makes sure that whatever is still present in /var/run/journal would get flushed there if it exists.