[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/core/settings.c

  * @v name		Configuration setting name
  * @v buf		Buffer to contain value
  * @v len		Length of buffer
- * @ret rc		Return status code
+ * @ret len		Length of formatted value, or negative error
  */
 int show_named_setting ( struct config_context *context, const char *name,
 			 char *buf, size_t len ) {
  * @v setting		Configuration setting
  * @v buf		Buffer to contain value
  * @v len		Length of buffer
- * @ret rc		Return status code
+ * @ret len		Length of formatted value, or negative error
  */
 static int show_string ( struct config_context *context,
 			 struct config_setting *setting,
 	option = find_dhcp_option ( context->options, setting->tag );
 	if ( ! option )
 		return -ENODATA;
-	dhcp_snprintf ( buf, len, option );
-	return 0;
+	return dhcp_snprintf ( buf, len, option );
 }
 
 /**
  * @v setting		Configuration setting
  * @v buf		Buffer to contain value
  * @v len		Length of buffer
- * @ret rc		Return status code
+ * @ret len		Length of formatted value, or negative error
  */
 static int show_ipv4 ( struct config_context *context,
 		       struct config_setting *setting,
 	if ( ! option )
 		return -ENODATA;
 	dhcp_ipv4_option ( option, &ipv4 );
-	snprintf ( buf, len, inet_ntoa ( ipv4 ) );
-	return 0;
+	return snprintf ( buf, len, inet_ntoa ( ipv4 ) );
 }
 
 /**
  * @v setting		Configuration setting
  * @v buf		Buffer to contain value
  * @v len		Length of buffer
- * @ret rc		Return status code
+ * @ret len		Length of formatted value, or negative error
  */
 static int show_int ( struct config_context *context,
 		      struct config_setting *setting,
 	if ( ! option )
 		return -ENODATA;
 	num = dhcp_num_option ( option );
-	snprintf ( buf, len, "%ld", num );
-	return 0;
+	return snprintf ( buf, len, "%ld", num );
 }
 
 /**

src/hci/commands/nvo_cmd.c

 
 	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

 
 	/* 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';
 	}	
 

src/include/gpxe/settings.h

 	 * @v setting		Configuration setting
 	 * @v buf		Buffer to contain value
 	 * @v len		Length of buffer
-	 * @ret rc		Return status code
+	 * @ret len		Length of formatted value, or negative error
 	 */
 	int ( * show ) ( struct config_context *context,
 			 struct config_setting *setting,
  * @v setting		Configuration setting
  * @v buf		Buffer to contain value
  * @v len		Length of buffer
- * @ret rc		Return status code
+ * @ret len		Length of formatted value, or negative error
  */
 static inline int show_setting ( struct config_context *context,
 				 struct config_setting *setting,