patches for DLZ update support

Mi Michael,

I've now gotten DLZ support for dynamic updates working quite
nicely. I have it setup against a Samba hosted AD domain with two
zones (the base zone, and the _msdcs zone).

The patches are a bit larger than the previous ones, but I hope you
will find them acceptable. Please let me know if there is something
you would like me to re-work.

The patches against 9.7.2-P2 are here:

  http://samba.org/tridge/bind9-patches/
  git://git.samba.org/tridge/bind9.git

I've also put patches against the rt22629 CVS branch in my home
directory on kechara as usual.

The approach I've taken with these patches is to keep the sdlz
interface as simple as possible, while exposing the ability to handle
updates. This is in keeping with the existing sdlz interface.

I've also expanded the dlz_dlopen driver to expose the new
methods. This makes it possible to write an external dlz driver that
supports dynamic updates.

Rob, if you have time, it would be great if you could look over the
patches as well and give me some comments. The key patches are the
ones starting with "dlz:" and "sdlz:".

I'm now working on an expanded version of the dlz stub driver to use
as an internal testsuite. Up to now I've been using the Samba4 dlz
driver for testing, but we really need a test suite that can be run as
part of make test within the bind9 tree.

Cheers, Tridge

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