Dynamic interface name substitution

Following #259, please add a way to use dynamic interface names in other interface-related objects and in templates.
Either a way to feed `${iface X}` rendered value to other objects, or change interface-related objects that expect interface name as an argument to treat a numeric argument the same way as `iface` (which probably would be cleaner from config perspective).

Start by reading issue #259 and tracing how `${iface X}` is rendered and how interface-related objects receive interface-name arguments. Determine whether the project should expose that rendered value or accept numeric arguments consistently, then verify that dynamic names work in the affected objects and templates.

Written by the indexing model from the issue text.