bind plugin: Add a new plugin for BIND 9.5 and 9.6 statistics.

Attached are patches for collectd-4.5.x and collectd-4.4.x series for a
plugin to collect statistics from a ISC Bind server.

ISC Bind introduces XML-based statistics which can be fetched via HTTP
starting with bind-9.5.0 and updated the XML structure with bind-9.6.0.
Both XML formats are supported by this plugin.

To enable access to these statistics, use the following configuration
block:

statistics-channels {
        inet localhost port 8053 allow { localhost; };
};

Suggestions on improving the grouping of statistics values are welcome,
currently the following groups are implemented:
  RRQueriesIn      type=dns_qtype
  Requests         type=dns_request
  QueryResults     type=dns_rcode
  Updates          type=dns_update
  ZoneMaintenance  type=dns_zops
  Resolver         type=dns_resolver

Bruno