multiplayer: two sync margins in the ini, and the session decides the trainsets

Two hardcoded constants decided how far a train could drift before it was put
back, and there was no way to change them short of a rebuild. They are settings
now:

    multiplayer.sync.running  1.5   // margin while the train is rolling
    multiplayer.sync.stop     0.2   // one precise correction once it stands

The split is what the two numbers are for. A moving train is left to its own
physics unless it is well out, because nudging it several times a second is
worse than the error; a standing one is worth getting right to the centimetre,
and it is done once and then left alone until it moves again - a consist that
has had its correction is marked settled, and clears that mark the moment it
rolls. Both travel in SERVER_HELLO: whatever a player has in their own ini, a
session runs to the host's margins, which is the only way the peers agree on
what "out of place" means.

And the trainsets. Every peer loads its own copy of the scenery - that is the
only reason joining takes seconds instead of minutes - but the copy on disk is
not the session: the host may have shunted a set apart, put one together, or be
running a scenery whose consists were edited since. So the composition travels
in a full snapshot, ahead of the positions: which coupler of which vehicle is
joined to which coupler of which other, and with what hoses and cables. A
joining peer breaks what should not be coupled, makes what should, and only
then gets told where everything is - so the per-consist repositioning works on
the session's trains rather than on the ones its own scenery file described.

The vehicles themselves do not travel, and cannot: a peer missing one gets a
line in the log saying so, not a model over the wire.

Uncoupling has to be forced here. TDynamicObject::Dettach() leaves the
mechanical coupler alone unless the buffers are pressed together, which is
right for a player pulling a lever and wrong for putting a peer's world in
order.

Protocol version 5, snapshot version 6.

network/message.h

@@ -78,6 +78,11 @@ struct server_hello : public message
 	uint32_t peer_id{ PEER_NONE };
 	// hand this back on a later connection to be recognised as the same participant
 	uint64_t session_token{ 0 };
+	// how far a train may drift from the session's idea of where it is before it is put
+	// back: while it rolls, and once it has stopped. the client takes these over from the
+	// server, so everybody in a session works to the same margins
+	double sync_running{ 1.5 };
+	double sync_stop{ 0.2 };
 
 	virtual void serialize(std::ostream &stream) const override;
 	virtual void deserialize(std::istream &stream) override;