add semaphore to cga_stream + use it in demos

c_os/user/demo/KeyboardDemo.cc

@@ -16,14 +16,16 @@ void KeyboardDemo::run() {
 
     kout << "Keyboard Demo: " << endl;
 
-    // kout.lock();
+    kout.lock();
     kout.clear();
+    kout << "Info: Die Keyboard Demo sperrt den Output Stream:\n"
+         << "      Wenn die Preemption Demo laeuft wird diese also erst\n"
+         << "      fortfahren wenn die Keyboard Demo wieder beendet ist." << endl;
+    kout << "\nInput: ";
+    kout.flush();
 
     while (true) {
         kout << listener.waitForKeyEvent();
         kout.flush();
     }
-
-    // kout.unlock();
-    scheduler.exit();
 }