Collapse debian branch onto master

The big rename from kafkacat to kcat

.. to keep credits use down.

Apache Software Foundation (ASF) trademark policies prohibits the use
of an Apache project name as part of a non-Apache project name.
In this case it was the use of `kafka` in `kafkacat` that had to be changed.

Apart from the name change there are no functional differences.
The now deprecated kafkacat.conf and KAFKACAT_CONF file and env vars
will continue to work for some time, but users should change to
kcat.conf and KCAT_CONF.