MINOR: hq-interop: decode response into HTX for backend side support

MINOR: hq-interop: decode response into HTX for backend side support

Implement transcoding of a HTTP/0.9 response into a HTX message.

HTTP/0.9 is a really simple substract of HTTP spec. The response does
not have any status line and is contains only the payload body. Response
is finished when the underlying connection/stream is closed.

A status line is generated to be compliant with HTX. This is performed
on the first invokation of rcv_buf for the current stream. Status code
is set to 200. Payload body if present is then copied using
htx_add_data().