Note the systemd-backend does not need the logpath at all;
Some defaults normalized (minimized configs, don't need to overwrite values in distribution-related path if equal).
Fixed mail.log path since in the default rsyslog configuration of debians the `mail.warn` is commented now (see `/etc/rsyslog.d/50-default.conf`: `#mail.warn -/var/log/mail.warn`).
Closes gh-1687