Add usr_modified field to users table, to be updated whenever the

user changes his personal info. This is intended to act as a serial
number when nodes ask for account info, but is also useful for
auditing.

sql/database-create.sql

@@ -862,6 +862,7 @@ CREATE TABLE users (
   uid varchar(8) NOT NULL default '',
   usr_created datetime default NULL,
   usr_expires datetime default NULL,
+  usr_modified datetime default NULL,
   usr_name tinytext,
   usr_title tinytext,
   usr_affil tinytext,