Template structure does dot use xinetd

[raresvis]

We usually ensure the availability of a service via xinetd. At the current stage, the template does not make use of it.

[raresvis]

As consequence, we need to think of a way of substituting the variables from common/activity.mk into xinetd.conf. I think the main concern is the PORT variable.

The first obvious solution is using sed. A solution that would require some minor tweaking is envsubst.