mod_systemd: Add IdleShutdown - number of seconds in idle-state after which

httpd is shutdown. This is useful in a combination with socket activation.
Add mod_systemd documentation.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1618579 13f79535-47bb-0310-9956-ffa450edef68