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.

Kernel/VirtualConsole.h

@@ -43,6 +43,8 @@ private:
     void escape$s(const Vector<unsigned>&);
     void escape$u(const Vector<unsigned>&);
 
+    void clear();
+
     const byte m_rows { 25 };
     const byte m_columns { 80 };
     byte m_cursor_row { 0 };