Replace mkTemplateOption by freeform options for service templates.

This is an extension of the previous "removal" of service templates, where we stopped using separate modules for options that should only be present in templates, and started using mkTemplateOption, that wrapped mkOption to make it behave differently when used in a context template.

Here, we go even further, and completely remove mkTemplateOption, implementing service options using a freeform module. The main disadvantage is that service template options are no longer documented (except instances, of course), but this can be handled by referring to service option defined by the machine module, and providing better examples.

The reason we are trying this compromise is that it can significantly reduce the code's complexity.