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/libsystemd/sd-bus/test-bus-creds.c

@@ -13,7 +13,7 @@ int main(int argc, char *argv[]) {
 
         test_setup_logging(LOG_DEBUG);
 
-        if (cg_unified_flush() == -ENOMEDIUM)
+        if (cg_unified() == -ENOMEDIUM)
                 return log_tests_skipped("/sys/fs/cgroup/ not available");
 
         r = sd_bus_creds_new_from_pid(&creds, 0, _SD_BUS_CREDS_ALL);