Networking used to be reachable only through the eu07.ini network.server /
network.client entries, and init_modes() refused to start at all unless a
scenery file was already known. That made it impossible to join a session
without knowing (and typing) the host's scenario up front, even though
SERVER_HELLO already carries the scenario name.
- add --host [address:port] and --connect address:port to the command line
(default port 7420, --host defaults to 0.0.0.0), plus --scenario/--vehicle
aliases and -h/--help; the existing ini settings keep working
- drop the init_modes() restriction: a client goes straight to the scenario
loader and waits there for the handshake, a host without -s still gets the
launcher and simply refuses joins until a scenario is up
- the loader screen reports the network state instead of sitting on a blank
progress bar
- a client that did not ask for a vehicle with -v starts as an observer
rather than failing to find the default EU07-424; the lobby of stage 2 will
take over the choice
- handshake carries build identification and the server answers an
impossible join with SERVER_REJECT and a readable reason instead of
dropping the socket silently; protocol version bumped to 3
- deserialize_message() no longer dereferences a null message on an unknown
message type
