Add trigger_mgr to iosource_mgr later during startup

This fixes a potential crash due to trigger_mgr getting shutdown earlier
than dns_mgr, and dns_mgr then trying to use it after it's been deleted.
This change forces the order of initialization/destruction in
iosource_mgr to cause dns_mgr to be deleted first.

src/Trigger.h

@@ -162,6 +162,8 @@ public:
 	Manager();
 	~Manager();
 
+	void InitPostScript();
+
 	double GetNextTimeout() override;
 	void Process() override;
 	const char* Tag() override { return "TriggerMgr"; }