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/DumpOgg.cxx

@@ -24,6 +24,7 @@ try {
 	const char *path = argv[1];
 
 	EventThread io_thread;
+	io_thread.Start();
 
 	const ScopeInputPluginsInit input_plugins_init(ConfigData(),
 						       io_thread.GetEventLoop());