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.