auth/ldap: MDL-9405 sync_users() can create duplicated users

auth/ldap: MDL-9405 sync_users() can create duplicated users

Merged from MOODLE_18_STABLE

If we are using auth_ldap_sync_users.php to synchronize our users, and we
have a database which is case-sensitive when doing comparisons (Postgres and
Oracle at least), and any of our users has the vale of the username attribute
in mixed-case (like 'John Smith'), we get duplicated users.

This is because we don't make sure the username attribute value is 'lowercased'
after we retrive it from the LDAP server and before we insert it into the
database.