Naming schema

This pull request follows [collectd naming schema](https://collectd.org/wiki/index.php/Naming_schema). Unfortunatelly I have no idea what is 'c' (category) key used for. In previous code I can see varnish in regexp. [Plugin page](https://collectd.org/wiki/index.php/Plugin:Varnish) does not mention anything unusual about naming schema.

Note:
This is not backwards compatible with collectd 4.x. Please see [this](https://collectd.org/wiki/index.php/Naming_schema#Plugin_instance_and_type_instance) for more information.