cgroup-util: add cg_is_delegated helper

Take is_delegated from cgroup-show.c, and make it a generic helper
function. This new helper will be used again in a later commit.

src/basic/cgroup-util.h

@@ -210,6 +210,8 @@ int cg_rmdir(const char *controller, const char *path);
 
 int cg_is_threaded(const char *path);
 
+int cg_is_delegated(const char *path);
+
 typedef enum  {
         CG_KEY_MODE_GRACEFUL = 1 << 0,
 } CGroupKeyMode;