OPTIM: haterm: use chunk builders for generated response headers

OPTIM: haterm: use chunk builders for generated response headers

hstream_build_http_resp() currently uses snprintf() to build the
status code and the generated X-req/X-rsp header values.

These strings are short and are fully derived from already parsed request
state, so they can be assembled directly in the HAProxy trash buffer using
`chunk_strcat()` and `ultoa_o()`.

This keeps the generated output unchanged while removing the remaining
`snprintf()` calls from the response-building path.

No functional change is expected.

Signed-off-by: Aleksandar Lazic <al-haproxy@none.at>