Update bind plugin to support the new XML v3 API.

This commit adds support for the new XML schema of Bind9 statistics-channels (version 3). This new schema is the default one in Bind9.9 (can be reverted to v2 with a `./configure` switch) and support for older versions has been dropped in Bind9.10.

Support for versions 1 and 2 remain in the collectd plugin.

Unfortunately, this new schema does not provide per-zone data any more, thus 'Zone' options like 
```
Zone "example.com/IN"
``` 
are ignored if schema version is 3.

Before this, a v3 schema was silently failed with only ``read-function of plugin `bind' failed.``.
Could you change this logic to the following to save one level of indentation?

```c
if (strncmp ("counter", (char *) child->name, strlen ("counter")) != 0)
  continue;
```