// The given username could be just the short "login" username or the full DN.
//
// When the username/DN is found, the full DN returned by the **server** is
// returned, otherwise the returned string is empty. The value returned here is
// the value sent by the LDAP server and is used in minio as the server performs
// LDAP specific normalization (including Unicode normalization).
//
// If the user is not found, err = nil, otherwise, err != nil.