Implement ldap_modify_batch.

This pull request implements the function `ldap_modify_batch`, which has recently [passed an RFC vote](https://wiki.php.net/rfc/ldap_modify_batch). The function allows precise specification of the modifications to perform on an entity in LDAP, which is necessary for some operations on some LDAP-accessible directory services.

The consensus on the mailing list seems to be to merge this into [5.4 and up](http://marc.info/?l=php-internals&m=139039793311536&w=2).