event/Loop: add flag alive

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

src/event/Thread.cxx

@@ -26,8 +26,11 @@
 void
 EventThread::Start()
 {
+	assert(!event_loop.IsAlive());
 	assert(!thread.IsDefined());
 
+	event_loop.SetAlive(true);
+
 	thread.Start();
 }
 
@@ -35,6 +38,9 @@ void
 EventThread::Stop() noexcept
 {
 	if (thread.IsDefined()) {
+		assert(event_loop.IsAlive());
+		event_loop.SetAlive(false);
+
 		event_loop.Break();
 		thread.Join();
 	}