Fix: [Network] don't show GameScript " (v0)" for old servers (#9507)

Old servers don't tell the GameScript they are running, so nothing
should be shown.
All values in NetworkGameInfo initialize as 0/empty, except for GS
version. Someone has to be different from the rest, I guess.

Remove: the concept of UnknownGRFs

These were filled with "<Unknown>" (before 8a2da49) and later their name would get filled via UDP requests to the server. These UDP packets do not exist anymore, so they will always remain "<Unknown>".
Remove that logic and just use the generic translated error GRF UNKNOWN string instead.

Codechange: track servers with a ServerAddress instead of a NetworkAddress

This allows future extensions to have different ways of referencing
a server, instead of forcing to use IP:port.

Remove: old server listing via Master Server

This removes all UDP from the game except for a local broadcast
to find LAN games.

So long Master Server, and tnx for all the fish!

Change: reworked the debug levels for network facility (#9251)

It now follows very simple rules:
0 - Fatal, user should know about this
1 - Error, but we are recovering
2 - Warning, wrong but okay if you don't know
3 - Info, information you might care about
4 -
5 - Debug #1 - High level debug messages
6 - Debug #2 - Low level debug messages
7 - Trace information

Codechange: use connection_string in favour of NetworkAddress (#9197)

We now resolve the connection_string to a NetworkAddress in a much
later state. This means there are fewer places constructing a NetworkAddress.

The main benefit of this is in later PRs that introduce different types
of NetworkAddresses. Storing this in things like NetworkGameList is
rather complex, especially as NetworkAddress has to be mutable at all
times.

Additionally, the NetworkAddress is a complex object to store simple
information: how to connect to this server.

Change: use TCP for everything except for master-server and initial server scan (#9130)

This means that pressing Refresh button and adding servers manually
now uses TCP.

The master-server and initial scan are still UDP as they will be
replaced by Game Coordinator; no need to change this now.

If we query a server that is too old, show a proper warning to the
user informing him the server is too old.

Codechange: use NetworkAddress instead of two host/port variables where possible

This also means we no longer need last_host/last_port, but can
just use a single last_joined setting.

Codechange: Replace custom mutex code with C++11 mutex'es.

A conforming compiler with a valid <mutex>-header is expected.
Most parts of the code assume that locking a mutex will never fail unexpectedly,
which is generally true on all common platforms that don't just pretend to
be C++11. The use of condition variables in driver code is checked.

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.