journalctl: allow running vacuum on remote journals, too

Right now the `systemd-journal-remote` service does not constrain its
resource usage (I just run out of space on my 100GB partition, for
example). This patch does not change that, but it at least makes it
possible to run something like:

  journalctl --directory /var/log/journal/remote --rotate --vacuum-size=90G

fixes #2376

Co-authored-by: Mike Auty <ikelos@gentoo.org>