core: keep supporting cgroup hybrid layout from v232 for live upgrades

core: keep supporting cgroup hybrid layout from v232 for live upgrades

v232's cgroup hybrid mode mounted v2 on /sys/fs/cgroup/systemd, which
unfortunately broke other tools which expect v1 there.  From v233 on, hybrid
mode instead mounts and uses v2 on /sys/fs/cgroup/unified and keeps
/sys/fs/cgroup/systemd on v1 for compatibility with external tools.  However,
to keep systemd live upgrades working, v233+ should be able to recognize v232
layout and keep using it.

This patch adds v232 hybrid mode support.  If v232 layout is detected,
cg_unified(SYSTEMD_CGRouP_CONTROLLER) keeps returning %true but
cg_hybrid_unified() returns %false.  This keeps process management on cgroup v2
but turns off the parallel layout.