Make /bin/clear work again.

After I made stdio buffered, we were dropping anything unflushed on exit.
Since /bin/clear just prints out some escape sequences without a newline,
the entire buffer was being discarded.

Also add VirtualConsole::clear() that handles clearing of background VC's.

LibC/entry.cpp

@@ -41,6 +41,9 @@ extern "C" int _start()
         goto epilogue;
     status = main(argc, argv);
 
+    fflush(stdout);
+    fflush(stderr);
+
 epilogue:
     Syscall::invoke(Syscall::SC_exit, status);