The runqueue list groups together all processes in a TASK_RUNNING state. When it comes to
grouping processes in other states, the various states call for different types of treatment.
Processes in a TASK_STOPPED or in a TASK_ZOMBIE state are not linked in specific lists.
There is no need to group them, because either the process PID or the process
parenthood relationships may be used by the parent process to retrieve the child
process.
