Univention Corporate Server 4.4 erratum 120 (bugfix)

This update addresses the following issues:
* Split low-level C implementation from Python binding to simplify transition
  to using `dh_python2` and adding Python 3 support.
* Unify pure-python version `debug2.py` again with C-version `debug.py`.
* Deprecate `function` class in favor of `trace` decorator.