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.

- Close the unused ends of pipes in the parent process.

- Don't stop reading from stdout if waitpid(2) shows that
  the process has exited, as there may still be pending
  output. We now call waitpid(2) only in vdp_fini.

- Close stdin as early as possible on VDP_END.

- Read stdout and stderr until they close.

- Don't use /bin/true and /bin/false in the tests, as
  they actually don't consume stdin, and may close
  stdin before we get to poll(2), resulting in POLLERR.
  This happened with varnishtest -jN, with j > about 5.

- Index VDPs with vdp and vcl pointers in the RB tree.
  This did not turn out to be a bugfix as I thought, but
  it should be more reliable, since VCL and object names
  may be identical in varnishtest -j tests.