Use new picoquic_iovect_t in alpn callback signature

[afrind]

This seems cleaner than casting in the alpn callback, but could be a breaking change.

[huitema]

We need to have two separate calls: the old one for compatibility, and a new one using the "v2" signature.

I would implement that by having an additional value in the "picoquic_quic_t" struct:

    picoquic_alpn_select_fn alpn_select_fn;
    picoquic_alpn_select_fn_v2 alpn_select_fn_v2;

and then modify the code in tls_api.c from:

    else if (quic->alpn_select_fn != NULL) {
        size_t selected = quic->alpn_select_fn(quic, params->negotiated_protocols.list, params->negotiated_protocols.count);

        if (selected < params->negotiated_protocols.count) {
            alpn_found = params->negotiated_protocols.list[selected].base;
            alpn_found_length = params->negotiated_protocols.list[selected].len;
            ptls_set_negotiated_protocol(tls, (const char *)params->negotiated_protocols.list[selected].base, params->negotiated_protocols.list[selected].len);
        }
    }

to:

    else {
        size_t selected = params->negotiated_protocols.count;
        if (quic->alpn_select_fn != NULL) {
            selected = quic->alpn_select_fn(quic, params->negotiated_protocols.list, params->negotiated_protocols.count);
        }
        else if (quic->alpn_select_fn_v2 != NULL) {
            selected = quic->alpn_select_fn_v2(quic, (picoquic_iovec_t*)&params->negotiated_protocols.list, params->negotiated_protocols.count);
        }

        if (selected < params->negotiated_protocols.count) {
            alpn_found = params->negotiated_protocols.list[selected].base;
            alpn_found_length = params->negotiated_protocols.list[selected].len;
            ptls_set_negotiated_protocol(tls, (const char *)params->negotiated_protocols.list[selected].base, params->negotiated_protocols.list[selected].len);
        }
    }

The current casting is not adequate -- it does break the windows builds.

[huitema]

OK. Just a minor suggestion in case the developer mixes the two APIs.