with vrt_ctx arguments.

This actually makes some of the macro-expansions in cache_vrt_var.c
simpler and saner...

even newer vrt_ctx argument.

and I've already forgot about them one important place already.

Introduce a "VRT_context", which is a container holding all the
bits we want to pass to VCL and VRT.

Implement it first for VRT_count(), which is, BTW, how I realized I
had forgotten to pass all the new arguments to user defined
functions.

Fix some test-cases to match, generally s/req.*/bereq.*/

NB: bereq.backend.healthy always fails right now.

to use busyobj.

Move VRT interface to cache_vrt.c rather than cache_vrt_var.c,
the latter is reserved for VCL variables.

Fixes #1285

are available.

Minor polish

threads a copy of the VCL.

Reduce move the meat of the inline VCL method functions to
a real function, before things get more complicated.

to vcl_backend_fetch{}, and vcl_response{} to vcl_backend_response{}.