core: rename cg_unified() to cg_all_unified()

A following patch will update cgroup handling so that the elogind controller
(/sys/fs/cgroup/elogind) 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 elogind
controller is.  In preparation, this patch renames cg_unified() to
cg_all_unified().

This patch doesn't cause any functional changes.