cgi-client: more robust buffer handling
cgi-client: more robust buffer handling

Instead of delivering the beginning of the response body from the
remaining header buffer, flush the buffer and resume sending from the
input's buffer at the right position.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy