supervisor: Fall back to a scanning /proc if CONFIG_PROC_CHILDREN is off
Without CONFIG_PROC_CHILDREN, task/TID/children is unavailable, so we can't use it to track child processes. Instead, we can just scan /proc for child processes by checking their PPid.
Fixes: https://gitlab.steamos.cloud/steamrt/tasks/-/issues/445
Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com