SDAP: add enterprise principal strings for user searches
    
    Unfortunately principal aliases with an alternative realm are stored in
    IPA as the string representation of an enterprise principal, i.e.
    name\@alt.realm@IPA.REALM. To be able to lookup the alternative
    principal in LDAP properly the UPN search filter is extended to search
    for this type of name as well.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>