... which determines if a Service defines the admin interfaces for
a Varnish Service that implements Ingress. We don't have to query
the Endpoints (which often don't exist yet when the Service is new),
because the service port definitions are in the Spec.

This means that the function doesn't have to return a possible
error, which simplifies the calling code. In particular,
filterVarnishIngSvcs(), which filters a slice of Services for those
for which isVarnishIngSvc() is true, does not have to pass along
the error return value. That in turn simplifies more calling code.