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: "language" field from server/client

The original idea was that people could find a server they could
talk in their native language on. This isn't really used in that
way. There are several reasons for removing this:

- the client also sends his "language" to the server, but nothing
is doing anything with this.
- flags are a bad way to represent languages, and over the years
we had several (rightfully) complaints about this.
- most servers have their language set to "All", and prefix the
servername with the language it is about. This is a much more
efficient way to do the same.

All in all, this feature should go back to the drawing board.
Maybe it could work in another form, but this form is not 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 r14730) -Codechange: remove the need for networkclientsockets and networkclientinfo structs to be in a contiguous piece of memory and put them in a pool.
-Note: 255 should really be enough for now... making it any more means network protocol bumps.