Reflect new C: close default in vcl_pipe

We now set Connection: Close on all piped backend requests, to avoid some of
the debugging pain seen over the last 2-3 years.

Basic problem is that request backend routing done in vcl_recv would not be
honored for subsequent requests after one was piped.

This was, at least for new users, a rather nasty surprise.

Original change in commit 880c3c2.