Move X-Forwarded-For handled into C code

Remove adding of X-Forwarded-For from builtin.vcl and put it into the
C code that runs before vcl_recv.  This makes it more likely that it's
properly set when the user VCL runs.

Fixes: #1454
Set Connection: close before calling vcl_pipe

It makes sense to default to setting Connection: close for vcl_pipe.
Do this before calling vcl_pipe so it's overrideable.

Fixes #1455