- 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
This makes it easier to run `./tools/check-messages.py` to correctly
assess the state of messages while developing Kea without having to
remove the premium directory.