(struct vrt_ctx).handling is a special animal: As seen by VGC, struct
vrt_ctx is const (and rightly so), yet VCL needs to have a way to
communicate up to core code the return value from subs.

So, handling was a (non-const) pointer into struct worker. This is all
good and well, with the minor drawback that VMODs would get to see it,
while we should make it very clear that they have to go through
VRT_handling() / VRT_handled() and know the rules (no reset of
handling).

As we now want to move also the vpi count into "the ctx", before adding
another unsigned pointer, we introduce struct wrk_vpi, provided by the
worker, but owned by vpi.

With this change, we thus formalize the vpi count requirement and also
remove the handling variable from vmods' direct line of sight.

This commit comprises the manual changes which either could not be or
were not worth being automated.

The next two commits contain a coccinelle script to do the rest of the
work, and the reult of it.