add semaphore to cga_stream + use it in demos

c_os/user/demo/PCSPKdemo.cc

@@ -1,9 +1,16 @@
 #include "user/demo/PCSPKdemo.h"
 
 void PCSPKdemo::run() {
+    kout.lock();
+    kout.clear();
     kout << "Playing..." << endl;
+    kout.unlock();
+
     (pcspk.*this->melody)();  // This syntax is confusing as hell
+
+    kout.lock();
     kout << "Finished" << endl;
+    kout.unlock();
 
     scheduler.exit();
 }