[console] Fix display of characters with top bit set

Inhibit implicit sign-padding of characters with the top bit set
(e.g. accented characters), which confuses the mucurses library by
colliding with the bits used to store character attributes and
colours.

Reported-by: Marc Delisle <Marc.Delisle@cegepsherbrooke.qc.ca>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/vsprintf.c

  * Call's the printf_context::handler() method and increments
  * printf_context::len.
  */
-static inline void cputchar ( struct printf_context *ctx, unsigned int c ) {
+static inline void cputchar ( struct printf_context *ctx, unsigned char c ) {
 	ctx->handler ( ctx, c );
 	++ctx->len;
 }

src/hci/mucurses/mucurses.c

  * @v wrap	wrap "switch"
  */
 void _wputc ( WINDOW *win, char c, int wrap ) {
-	_wputch ( win, ( c | win->attrs ), wrap );
+	_wputch ( win, ( ( ( unsigned char ) c ) | win->attrs ), wrap );
 }
 
 /**