varnish plugin: add missing fields

This commit adds a large number of fields exported by the varnish stats
interface which have not previously been collected. A number are still missing,
as it is not immediately obvious which stat group they should go in:

    accept_fail
    client_drop_late

and, from Varnish 3.X:

    vmods
    n_gzip
    n_gunzip

In addition, there are a number of fields which were added at the end of the 2.X
series, but in the absence of a way of detecting whether we're compiling against
2.0.X or 2.1.X, are necessarily disabled when compiling against 2.X:

    dir_dns_lookups
    dir_dns_failed
    dir_dns_hit
    dir_dns_cache_full
    backend_retry
    fetch_1xx
    fetch_204
    fetch_304