Use always only lowercase version of username.

When using authentication module (ldap, smb, pop etc...) and postgres-database multiple moodle
users were created for usernames like 'Username' 'username' 'userName'

user/edit.php

@@ -43,7 +43,7 @@
 /// If data submitted, then process and store.
 
 	if ($usernew = data_submitted()) {
-
+        $usernew->username = strtolower($usernew->username);
         $usernew->firstname = strip_tags($usernew->firstname);
         $usernew->lastname  = strip_tags($usernew->lastname);