Remove task descriptors when tasks complete

Calling norns(ctl)?_error() to query the status of a running task will
release its descriptor in the server if the task already completed
(NORNS_EFINISHED or NORNS_EFINISHEDWERROR). Since norns(ctl)?_wait()
internally queries the task status to check whether a task has
completed, we cache the retrieved task_status into the iotask_t user
descriptor so that it can be retrieved by a subsequent
norns(ctl)?_error() call.