[Settings] show_setting() functions return snprintf()-style length.

show_setting() and related functions now return an "actual length" in the
style of snprintf().  This is to allow consumers to allocate buffers large
enough to hold the formatted setting.

src/hci/commands/nvo_cmd.c

@@ -28,7 +28,7 @@ static int show_exec ( int argc, char **argv ) {
 
 	dummy_context.options = ugly_nvo_hack->options;
 	if ( ( rc = show_named_setting ( &dummy_context, argv[1], buf,
-					 sizeof ( buf ) ) ) != 0 ) {
+					 sizeof ( buf ) ) ) < 0 ) {
 		printf ( "Could not find \"%s\": %s\n",
 			 argv[1], strerror ( -rc ) );
 		return 1;

src/hci/tui/settings_ui.c

@@ -118,7 +118,7 @@ static void load_setting ( struct setting_widget *widget ) {
 
 	/* Read current setting value */
 	if ( show_setting ( widget->context, widget->setting,
-			    widget->value, sizeof ( widget->value ) ) != 0 ) {
+			    widget->value, sizeof ( widget->value ) ) < 0 ) {
 		widget->value[0] = '\0';
 	}