These types are all subclasses of NMObject. These instances are commonly
created by NMClient itself. It makes no sense that a user would
instantiate the type. Much less does it make sense to subclass them.

Hide the object and class structures from public API.

This is an API and ABI break, but of something that is very likely
unused.

This is mainly done to embed the private structure in the object itself.
This has benefits for performance and debugability. But most
importantly, we can obtain a static offset where to access the private data.
That means, we can use the information to access the data pointer
generically, as we will need later.

This is not done for the internal types NMManager, NMRemoteSettings,
and NMDnsManager. These types will be dropped later.