Allow independently stop KRaft controllers or brokers

Some users run KRaft controllers and brokers on the same machine (not containerized, but through tarballs, etc).  Prior to KRaft, when running ZooKeeper and Kafka on the same machine, users could independently stop the ZooKeeper node and Kafka broker since there were specific shell scripts for each (zookeeper-server-stop and kafka-server-stop, respectively).

However in KRaft mode, they can't stop the KRaft controllers independently from the Kafka brokers because there is just a single script that doesn't distinguish between the two processes and signals both of them. We need to provide a way for users to kill either controllers or brokers.