fixing 691: inconsistent foreign keys in usr

htdocs/lib/db/install.xml

@@ -114,6 +114,7 @@
                 <KEY NAME="primary" TYPE="primary" FIELDS="id" />
                 <KEY NAME="institution" TYPE="foreign" FIELDS="institution" REFTABLE="institution" REFFIELDS="name"/>
                 <KEY NAME="lastauthinstancefk" TYPE="foreign" FIELDS="lastauthinstance" REFTABLE="auth_instance" REFFIELDS="id" />
+                <KEY NAME="authinstancefk" TYPE="foreign" FIELDS="authinstance" REFTABLE="auth_instance" REFFIELDS="id" />
             </KEYS>
             <INDEXES>
                 <INDEX NAME="usernameuk" UNIQUE="true" FIELDS="username,authinstance"/>

htdocs/lib/db/upgrade.php

@@ -386,6 +386,10 @@ function xmldb_core_upgrade($oldversion=0) {
         $key->setAttributes(XMLDB_KEY_FOREIGN, array('lastauthinstance'), 'auth_instance', array('id'));
         add_key($table, $key);
 
+        $key   = new XMLDBKey("authinstancefk");
+        $key->setAttributes(XMLDB_KEY_FOREIGN, array('authinstance'), 'auth_instance', array('id'));
+        add_key($table, $key);
+
         $record = new stdClass();
 
         $record->name            = 'mahara';