New field "guid" added to table user.

This can be userd to store "global unique identifier" from external databases
like Windows SID or eDirectorys guid . This will keep upcoming user syncronization
feature reliable in case of renamed users.
Mysql files are untested.
php -l mysql.php does not show any syntax errors, so files
should work.

lib/db/postgres7.sql

@@ -198,6 +198,7 @@ CREATE INDEX prefix_cache_text_md5key_idx ON prefix_cache_text (md5key);
 CREATE TABLE prefix_user (
    id SERIAL PRIMARY KEY,
    auth varchar(20) NOT NULL default 'manual',
+   guid varchar(128) default NULL,
    confirmed integer NOT NULL default '0',
    deleted integer NOT NULL default '0',
    username varchar(100) NOT NULL default '',
@@ -235,6 +236,8 @@ CREATE TABLE prefix_user (
    CONSTRAINT prefix_user_username_uk UNIQUE (username)
 );
 
+CREATE INDEX prefix_user_auth_guid_idx ON prefix_user (auth, guid);
+
 CREATE TABLE prefix_user_admins (
    id SERIAL PRIMARY KEY,
    userid integer NOT NULL default '0'