We want to avoid deadlock situations and right now if there is a process
running with the same PID that is not our process (e.g. after a reboot)
we might be waiting forever to run the cron again. So to make this
transparent to the user, we at least post a warning into the logfile.

Preserve body structure in send_notification_email

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

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

Update letsencrypt.inc.php as per the discussion at ispconfig/ispconfig3#5030. Already pr'ed and merged in master via ispconfig/ispconfig3!778. Perhaps can be merged with stable too?

Add description text for logging options.