WIP: support multiple addresses for network interfaces

[schrd]

The idea is to use the kind_of attribute of a property to differentiate between properties that need array values to be joined by " " and properties that need to be written multiple times in the unit file

This patch is meant to be backwards compatible for other properties.

However, a better approach would be to do it the other way around and to introduce a class JoinedArray and then to do the " " join in options_value only for those properties that include the JoinedArray in their kind_of attribute

If you like the idea, let me know and I will do it that way.