Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b

Support the task-scoped invocation in vcl_deliver.

Use a VSTAILQ for the setenv list. VSLIST pushes only to the head,
so setenv() calls were in reverse order compared to the VCL calls.
We want later invocations of .setenv() overwrite the value of previous
invocations for the same variable (if overwrite is true). So use
a tail queue and push to the tail.

But we might go back to it when we add the VFP, since much of the
code for I/O with the child process should be reuasable.