Support HTTP responses that neither specify a Content-Length header

Support HTTP responses that neither specify a Content-Length header
nor chunked encoding.  We don't know the length of the HTTP body in
this case, so it only works for single-pass HTTP responses without
subsequent HTTP response headers in the stream.  You can still enforce
the Content-Length header with an "expect" rule.

For example, this fixes response handling from undeadly.org (thttpd)
if relayd is running as a transparent HTTP proxy.