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.

Userland/clear.cpp

@@ -1,8 +1,9 @@
-#include <LibC/stdio.h>
+#include <stdio.h>
 
 int main(int, char**)
 {
     printf("\033[3J\033[H\033[2J");
+    fflush(stdout);
     return 0;
 }