Feature: allow the use of STUN to connect client and server together

This method doesn't require port-forwarding to be used, and works for
most common NAT routers in home setups. But, for sure it doesn't work
for all setups, and not everyone will be able to use this.

Change: [Network] lower TCP connect() timeout to 3s (#9112)

Currently we use default OS timeout for TCP connections, which
is around 30s. 99% of the users will never notice this, but there
are a few cases where this is an issue:

- If you have a broken IPv6 connection, using Content Service is
first tried over IPv6. Only after 30s it times out and tries
IPv4. Nobody is waiting for that 30s.
- Upcoming STUN support has several methods of establishing a
connection between client and server. This requires feedback
from connect() to know if any method worked (they have to be
tried one by one). With 30s, this would take a very long time.

What is good to mention, is that there is no good value here. Any
value will have edge-cases where the experience is suboptimal. But
with 3s we support most of the stable connections, and if it fails,
the user can just retry. On the other side of the spectrum, with 30s,
it means the user has no possibility to use the service. So worst case
we annoy a few users with them having the retry vs annoying a few
users which have no means of resolving the situation.

Add: support for emscripten (play-OpenTTD-in-the-browser)

Emscripten compiles to WASM, which can be loaded via
HTML / JavaScript. This allows you to play OpenTTD inside a
browser.

Co-authored-by: milek7 <me@milek7.pl>

Fix: [Cygwin] Fix missing AI_ADDRCONFIG declaration

Fixes the following complaints:

src/network/core/address.cpp: In member function 'const sockaddr_storage* NetworkAddress::GetAddress()':
src/network/core/address.cpp:134:55: error: 'AI_ADDRCONFIG' was not declared in this scope
this->Resolve(this->address.ss_family, SOCK_STREAM, AI_ADDRCONFIG, nullptr, ResolveLoopProc);

Signed-off-by: Joe Stringer <joe@wand.net.nz>

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.

Fix: warnings when compiling for a recent version of Haiku

This might break older Haiku versions, but it is hard to tell.

Remove: BeOS support (deprecated by Haiku)

In 10 years there is no commit to change how BeOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), BeOS is no longer
support. SDL2 suggests to use Haiku instead of BeOS.

Remove: MorphOS / AmigaOS support

In 10 years there is no commit to change how MorphOS works, and we
have no active maintainer for it. It is unlikely it works in its
current state (but not impossible).

With the arrival of SDL2 (and removal of SDL), MorphOS is no longer
support. There is an SDL2 port for MorphOS, but it is not maintained
by upstream SDL2, and nobody can currently test it out.

If anyone wants to re-add MorphOS, please do (revert this patch,
fix the problems, and create a Pull Request). If you need any help
doing so, let us know! It is not that we don't like MorphOS, it is
that we don't have anyone fixing the problems :(

(svn r11480) -Codechange: Rename the function ALIGN fitting to the naming style
This fixes also FS#1450

(svn r8674) [PSP] -Add: added network code for PSP, based on the work of Turulo
-Add: added general header-inclusing for PSP

(svn r7767) -Fix r7751: [OSX] nameclash in includes. ALIGN was defined in both a library and macros.h
we undefines the library one since we don't need that one anyway

(svn r7759) -Merge: makefile rewrite. This merge features:
- A proper ./configure, so everything needs to be configured only once, not for every make.
- Usage of makedepend when available. This greatly reduces the time needed for generating the dependencies.
- A generator for all project files. There is a single file with sources, which is used to generate Makefiles and the project files for MSVC.
- Proper support for OSX universal binaries.
- Object files for non-MSVC compiles are also placed in separate directories, making is faster to switch between debug and release compiles and it does not touch the directory with the source files.
- Functionality to make a bundle of all needed files for for example a nightly or distribution of a binary with all needed GRFs and language files.

Note: as this merge moves almost all files, it is recommended to make a backup of your working copy before updating your working copy.