- use cleanup macros everywhere.

- In particular use nm_auto_clear_variant_builder to free the
  GVariantBuilder in the error cases. Note that the error cases
  anyway are asserted against, so during a normal test run there
  was no leak. But we should not write software like that.

- use nm_utils_strsplit_set_with_empty() instead of g_strsplit_set().
  We should use our variant also in unit-tests, because that way the
  function gets more test coverage. And it likely performs better
  anyway.