remove default backend (systemd) - too dangerous for all jails, because it's hardly to find an error if some jail mistakenly start to monitor journal instead of logfile (even if it exists), but will silently find nothing;
sshd jail disabled by default - user have to enable jails in jail.local or jail.d
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
