changed COPR repo

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.

Added simple windows build directions

(I am the packager here fwiw)

 * refactoring and fixes of the original PR
 * added key_error, payload_error optional fields to JSON message
   envelope to indicate deserialization errors.
 * prepare for future Avro producer when we (or preferably avro-c)
   figure out how to convert JSON to Avro.
 * fix librdkafka version in bootstrap.sh
 * make Dockerfile rely more on bootstrap.sh
 * use edenhill fork of yajl to allow embedding the avro-to-json
   deserialized message key/value directly in the JSON envelope with -J.