Clean callbacks for the host parsing.

- do not rely on global field, but pass the parameter to the callbacks
- do not show the dynar containing the callbacks to the public, but
  provide a specific function to add a callback not needing to see the
  dynar

This breaks some tests, but I need to split my changes to see when I
improve the situation, and revert stuff when I degrade the situation.
I start to fix the tests right away.