Codechange: [Network] split CloseSocket and CloseConnection more clearly (#9261)

* Codechange: [Network] split CloseSocket and CloseConnection more clearly

- CloseSocket now closes the actual OS socket.
- CloseConnection frees up the resources to just before CloseSocket.
- dtors call CloseSocket / CloseConnection where needed.

Codechange: remove pointless close call due to resolving virtual functions statically in destructors

In the destructors of many of the network related classes Close() is called, just like the
top class in that hierarchy. However, due to virtual functions getting resolved statically
in the destructor it would always call the empty Close() of the top class.
Document the other cases where a virtual call is resolved statically.

Fix: [Network] clients leaving because of broken connections was not broadcasted (#9238)

The code mixed up "client has quit but we already told everyone"
with "client lost connection, handle this".

Split up those two signals:
- CLIENT_QUIT means we told everyone and the connection is now dead
- CONNECTION_LIST means we should tell everyone we lost a client

Codechange: move all NetworkGameInfo related functions to a single file

It currently was a bit scattered over the place. Part of
NetworkGameInfo is also the GRF Identifiers that goes with it.

Remove: ENABLE_NETWORK switch

This switch has been a pain for years. Often disabling broke
compilation, as no developer compiles OpenTTD without, neither do
any of our official binaries.

Additionaly, it has grown so hugely in our codebase, that it
clearly shows that the current solution was a poor one. 350+
instances of "#ifdef ENABLE_NETWORK" were in the code, of which
only ~30 in the networking code itself. The rest were all around
the code to do the right thing, from GUI to NewGRF.

A more proper solution would be to stub all the functions, and
make sure the rest of the code can simply assume network is
available. This was also partially done, and most variables were
correct if networking was disabled. Despite that, often the #ifdefs
were still used.

With the recent removal of DOS, there is also no platform anymore
which we support where networking isn't working out-of-the-box.

All in all, it is time to remove the ENABLE_NETWORK switch. No
replacement is planned, but if you feel we really need this option,
we welcome any Pull Request which implements this in a way that
doesn't crawl through the code like this diff shows we used to.

(svn r8479) -Fix (r8459): Silenced VC warning C4099: 'Packet' : type name first seen using 'class' now seen using 'struct'
-Codechange: 'typedef struct Packet' changed to 'struct Packet'