The solution is based on clusterfuzzlite, libfuzzer, and oss-fuzz
technologies.
- Add the .clusterfuzzlite directory.
- Add the fuzz CI stage and fuzzing CI jobs.
- Add the fuzzing targets in the fuzz directory.
- Document fuzzing in doxygen.
- generate kea-messages.rst automatically on make -C doc/sphinx
- add disclaimer header that debug-messages.rst and kea-messages.rst are
generated
- minor changes to debug level variable names
- do not write section "Messages printed" in tools/check-messages.py if
there are no messages
