Add new file 100-monitor_kernel_version.inc.php to stable-3.1 in relation to recently merged commits of ispconfig/ispconfig3!790 and ispconfig/ispconfig3!791.

There is a fix to the filename for master submitted via commit ispconfig/ispconfig3!804 that is still pending.

every cron execution. The logfile can be disabled by using -NoLog

instead we are now using dynamic purging of the logs.

add functions to 550-bind_dnssec.inc.php (see https://www.howtoforge.com/community/threads/dnssec-problem-expire-too-soon-ispconfig-3-1.74682/#post-351381)

DNSSEC-Cronjob should only run on ispconfig master as we are working with datalog and not accessing things directly

https://www.howtoforge.com/community/threads/quota-warnings-notifications.73747/#post-347405

- added 'certbot' to which command (if letsencrypt is not installed as system package, but certbot is)