core: rename cg_unified() to cg_all_unified()

core: rename cg_unified() to cg_all_unified()

A following patch will update cgroup handling so that the systemd controller
(/sys/fs/cgroup/systemd) can use the unified hierarchy even if the kernel
resource controllers are on the legacy hierarchies.  This would require
distinguishing whether all controllers are on cgroup v2 or only the systemd
controller is.  In preparation, this patch renames cg_unified() to
cg_all_unified().

This patch doesn't cause any functional changes.