[settings] Add setting deletion (Ctrl-D) interface

Make Ctrl-D delete a setting, because the Text User Interface (tui)
previously provided no way to delete a setting.  Also, update the
on-screen instructions to describe the new feature.  Deleting settings
is especially important for settings stored in precious nonvolatile
storage.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/hci/tui/settings_ui.c

 		      "Ctrl-C - discard changes" );
 	} else {
 		msg ( INSTRUCTION_ROW,
+		      "Ctrl-D - delete setting" INSTRUCTION_PAD
 		      "Ctrl-X - exit configuration utility" );
 	}
 }
 				if ( next > 0 )
 					reveal ( &widget, --next ) ;
 				break;
+			case CTRL_D:
+				delete_setting ( widget.settings,
+						 widget.setting );
+				select_setting ( &widget, next );
+				draw_setting ( &widget );
+				break;
 			case CTRL_X:
 				return 0;
 			default: