basic/cgroup-util: let cgroup_unified_flush() return the detected hierarchy

This avoid the use of the global variable.

Also rename cgroup_unified_update() to cgroup_unified_cached() and
cgroup_unified_flush() to cgroup_unified() to better reflect their new roles.

src/core/cgroup.c

@@ -2839,7 +2839,7 @@ int manager_setup_cgroup(Manager *m) {
         if (r < 0)
                 return log_error_errno(r, "Cannot find cgroup mount point: %m");
 
-        r = cg_unified_flush();
+        r = cg_unified();
         if (r < 0)
                 return log_error_errno(r, "Couldn't determine if we are running in the unified hierarchy: %m");