[th/utils-api-rename] rename some internal API

more renaming.

It's important that the new names don't accidentally existed before renaming, so that accidental use of an old name causes a compiler error. That's the case here.

the nm_utils_ prefix is not useful, if everything is an "util".

The original idea (for me) to use the "utils" prefix was that the prefix somehow should be the topic, and the topic should match the .c/.h source file where it resides. For example, nm_manager_* API you'd find in src/core/nm-manager.h.

But glib-aux and similar are mostly a heap of miscellaneous utilities, and it's not sensible to add for each topic it's distinct header/source file. In particular, because the topics are small.