Revert "event/Thread: start the thread in the constructor"

This reverts commit b49cfe96f4.  It was
a bad idea because it broke signal handlers.  I need to find a better
way to fix io_uring intialization.

test/run_input.cxx

@@ -130,6 +130,7 @@ public:
 	explicit GlobalInit(Path config_path)
 		:config(AutoLoadConfigFile(config_path))
 	{
+		io_thread.Start();
 	}
 };