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/basic/cgroup-util.h

@@ -258,7 +258,10 @@ bool cg_ns_supported(void);
 int cg_all_unified(void);
 int cg_hybrid_unified(void);
 int cg_unified_controller(const char *controller);
-int cg_unified_flush(void);
+int cg_unified_cached(bool flush);
+static inline int cg_unified(void) {
+        return cg_unified_cached(true);
+}
 
 bool cg_is_unified_wanted(void);
 bool cg_is_legacy_wanted(void);