[collectd] varnish plugin updates

[Marc Fournier]

Hello,

Here are a couple of patches I would like to suggest for inclusion before the
5.0 release.

The first one adds the possibility to collect all the remaining stats defined
in varnish's stat_field.h. Rationale:
 - I wanted to make graphs out of varnish's cache purge activity, and couldn't
   because we arbitrarily decided that it wasn't useful to collect these
   values.
 - more stats were added to stat_field.h in varnish release 2.1.4
   (backend_retry, dir_dns_lookups, dir_dns_failed, dir_dns_hit,
   dir_dns_cache_full ) and in 2.1.5 (fetch_1xx, fetch_204, fetch_304).
 - the nagios plugin for varnish is unnecessarily complicated to build, and
   doesn't seem actively maintained. Using collectd-nagios instead makes
   much more sense to me.

The 2 other patches are code cleanup and a minor bugfix. You'll also find
these patches here: https://github.com/mfournier/collectd/tree/varnish-plugin-updates

Cheers,
Marc