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 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.

Missed .inc in filename 100-monitor-kernel-version.php

nginx: allow cgi execution under protected folder
nginx: allow protected folder under cgi-bin folder

Add new file to display current kernel info. Related to comment !789 (comment 63987)

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.