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/nspawn/nspawn.c

@@ -4720,7 +4720,7 @@ static int run(int argc, char *argv[]) {
         if (r < 0)
                 goto finish;
 
-        r = cg_unified_flush();
+        r = cg_unified();
         if (r < 0) {
                 log_error_errno(r, "Failed to determine whether the unified cgroups hierarchy is used: %m");
                 goto finish;