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 API version is 3.