rc-cgroup.sh: avoid process substitution for cgroup_get_pids
rc-cgroup.sh: avoid process substitution for cgroup_get_pids

This should make cgroup_cleanup work successfully since cgroup_get_pids
no longer uses a subshell.

This fixes #396.
This fixes #397.

OpenRC/openrc,OpenRC/openrc,williamh/openrc,williamh/openrc,williamh/openrc,OpenRC/openrc