configure: add varnish4 presence detection
varnish: bare minimum changes to support varnish4
varnish: adapt to metrics renames & deprecations in varnish4

Summary of changes:
- connections: "accepted" & "dropped" are now found in the session section
- dirdns: doesn't exist in varnish 4 anymore
- objects: "n_obj*" were removed from varnish 4
- ban: "obj", "req" & "completed" bans were added. As a lot of new
  "tested" metrics are available, so we only collectd the overall total
  now
- session: metrics from "connections" and "threads" categories were
  moved here
- struct: "n_sess*" were removed
- totals: new, more detailed *bytes metrics were made available
- threads: new metrics were made available. "queued" was moved the
  session section.
varnish: add a couple of metrics available both in V3 & V4
varnish: monitor 2 purge-relatad metrics added in V4
varnish: monitor VSM usage stats, made available in V4
varnish: correct an erroneous type

Affects Varnish 3 only.
varnish: manpage & example cleanup

Consitently alphabetically ordered options and ensured all version
limitations are mentioned.
varnish: add some detail & references to manpage
Merge pull request #783 from mfournier/varnish4

Add support for varnish 4.x