system-source: Prefer the fowarding socket when running under systemd

When running under systemd, and syslog forwarding is enabled, prefer it over the journal source. Otherwise the Journal will keep complaining that forwarding messages to syslog does not work (since there is nothing listening there, it does fail).

This is not 100% ready yet, I have yet to test it on a real system with forwarding disabled. The unit file may need changes too, to not depend on syslog.socket. (If syslog.socket is started, `/run/systemd/journal/syslog` will be present, whether forwarding is enabled or not, which defeats our purpose.)