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.