* Listeners really do need these, where defined in the schemas, so state
that they're mandatory.
* REALLY emphasise that where an `Odyssey` flag isn't in `LoadGame` Senders
MUST NOT send it with a `false` value.
Although pending addition of header->gameversion will aid Listeners,
currently this could give the misleading impression that a 3.8 Horizons
client is a 4.0 Horizons client.
This includes the new `4.0 Horizons` client, which changes the semantics
of what the `Odyssey` flag in the `Fileheader` means. It now indicates if
the client is a 4.0+ one, not if the player is utilising Odyssey DLC.
This causes issues, at the least, with compressed messages that 'look' like
they decompressed body is form-encoded. 18385 messages in the last month
rejected due to this.
No actually valid form-encoded messages in that time frame.
* Most of it is once more in schemas/README.
* But keeping the horizons/odyssey flags in docs/Developers.md, as that's
more about how you handle them in code.
