Fix system log rotation

[matze]

Recent versions of Debian derivates do not use the SysV init-style any more, hence some of our older scripts do not work as intended in such an environment any more:

From: Cron Daemon <root@*>
Date: Wed, Jun 8, 2016 at 1:00 AM
Subject: Cron <root@*> /usr/sbin/logrotate /etc/logrotate.conf
To: root@*

logrotate_script: 2: logrotate_script: invoke-rc.d: not found
error: error running non-shared postrotate script for /var/log/syslog of '/var/log/syslog
'

These circumstances are well-known and documented though, so fixing should be a pretty straightforward task.