Fix bind DLZ module to prevent zone transfers for everyone

The Bind DLZ module was accepting all zone transfer requests, as long as they belongs to the domain, which was seen as a security concern by some users. This issue is referenced in bugzilla here: https://bugzilla.samba.org/show_bug.cgi?id=9634 for bug reference and suggested changes.

As suggested in this ticket, I have modified the module so that it takes a new option from smb.conf listing the authorized IPs. If the option is not set, the behaviour is not changed: the module will accept all requests.