init: trigger shutdown directly from builtins

Especially now that property_service is a thread, there may be some
delay between when init sets sys.powerctl and when the main thread of
init receives this and triggers shutdown.  It's possible that
outstanding init commands are run during this gap and that is not
desirable.

Instead, have builtins call TriggerShutdown() directly, so we can be
sure that the next action that init runs will be to shutdown the
device.

Test: reboot works
Test: reboot into recovery due to bad /data works
Change-Id: I26fb9f4f57f46c7451b8b58187138cfedd6fd9eb
diff --git a/init/init.h b/init/init.h
index c7918e7..d884a94 100644
--- a/init/init.h
+++ b/init/init.h
@@ -31,7 +31,7 @@
 Parser CreateParser(ActionManager& action_manager, ServiceList& service_list);
 Parser CreateServiceOnlyParser(ServiceList& service_list);
 
-void EnterShutdown(const std::string& command);
+void TriggerShutdown(const std::string& command);
 
 bool start_waiting_for_property(const char *name, const char *value);