auth/ldap: Better warnings and handling of updaging multi-source LDAP fields
auth/ldap: Better warnings and handling of updaging multi-source LDAP fields

 + Raise a warning if the auth_ldap_get_entries for one user record
   returns more than one record (!?)
 + Resolved some subtle bugs when updating a remote LDAP repo from user
   updates. The logic is now simpler too.
 + Simplified the code that compares old/new values

Merged from MOODLE_15_STABLE

dilawar/moodle