[collectd] Varnish plugin

[Jerome Renard]

Hi Florian,

I started to modify the plugin according the your comments you sent me
a few days ago.

Can you please tell me if that looks better to you now ?

The diff can be seen here :
- http://github.com/jeromer/collectd/commit/f6e61e888a5da0c910b8ab18997d0dfd21615266

Note :
I did not have time to implement a configuration callback yet, I'll do
it as soon as I can

Thanks in advance for your feedback

--
Jérôme