Provide API for detecting if running under Systemd for non-spring apps

[edgecase14]

Providing a similar function to internal APIs SystemdUtilities.isUnderSystemd() or SystemdNotify.usable() would be helpful for non-Spring/non-Tomcat apps. I am actually running Jetty Embedded but am happy with a plain non-integrated implementation.

I noticed the Systemd.logStatus() method which is helpful, but I can't read the result programmatically.

Other than that, this is a great piece of work, thanks!