Improved the code that generates the ssh keys for clients and shell users and reorganized it into a central function.

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

- disabled database logging of sql reconnect errors, fixed #3974

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

add optional arrays for service-name to function system->getinitcommand and also a parameter to check if the current service exists (mandatory if you use an array)

Removed ubuntu1604.lib.php which is not required anymore.

- Introduced confid in $distver array to separate the config and class for a distribution in the installer.
- Cleaned up the dist detection.