dnsdist: Fix invalid proxy protocol payload on a DoH TC to TCP retry

dnsdist: Fix invalid proxy protocol payload on a DoH TC to TCP retry

dnsdist forwards incoming DoH queries to its backend over UDP, and
retry over TCP if the response is truncated (TC=1).
When the proxy protocol is used between dnsdist and its backend, the
second query, over TCP, needs to take into account that the proxy
protocol payload has already been handled. This was not properly done
in that exact case because the proxy protocol payload length was not
propagated to the code handling the TCP communication, leading to
the query ID being edited at the wrong offset in the packet and thus
to an invalid proxy protocol payload.