xinetd not supported anymore by distributions

Since RedHat 9 the software xinetd is not shipped along anymore. The functionality of xinetd can and should be done now with systemd. All distributions which have systemd will stop using xinetd. Because of this Observium should switch to systemd as well.

Here is an example how the systemd config files should look like:

https://portal.microfocus.com/s/article/KM000005429?language=en_US

If you want I can create them for Observium and attach them to this bug report.

For sure the documentation at https://docs.observium.org/unix_agent/ needs to be updated as well.