varnish plugin: add missing fields

This is a resurrection of a series of patches submitting to the mailing list
over two years ago, in March 2011[1]. It adds support to the varnish plugin for
the collection of a whole bunch of stats exposed via the varnish API but
previously not collected.

[1]: http://mailman.verplant.org/pipermail/collectd/2011-March/004405.html

As the commit message explains, there are still a few stats that we're not
collecting, because I'm not entirely sure where to put them, and unfortunately
when compiling against varnish 2.1.X some stats exposed by varnish won't be
exposed by the plugin, simply because the configure scripts don't currently
distinguish between 2.0.X and 2.1.X.

This is my first patch submission to collectd, so do please let me know if there
are docs I should update, or if there are any issues (semantic or stylistic)
with the patch.