Remove: DOS support

In 10 years there was no active development on DOS. Although it
turned out to still work, the FPS was very bad. There is little
interest in the current community to look into this.

Further more, we like to switch to c++11 functions for threads,
which are not implemented by DJGPP, the only current compiler
for DOS.

Additionally, DOS is the only platform which does not support
networking. It is the reason we have tons of #ifdefs to support
disabling networking.

By removing DOS support, we can both use c++11 functions for threads,
and remove all the code related to disabling network. Sadly, this
means we have to see DOS go.

Of course, if you feel up for the task, simply revert this commit,
and implement stub c++11 functions for threads and stub functions
for networking. We are more than happy to accept such Pull Request.

README.md

@@ -152,7 +152,6 @@ OpenTTD has been ported to several platforms and operating systems. It should
 not be very difficult to port it to a new platform. The currently working
 platforms are:
 
-- DOS (Allegro)
 - FreeBSD (SDL)
 - Haiku (SDL)
 - Linux (SDL or Allegro)
@@ -555,17 +554,6 @@ Use '`make`', but do a '`./configure`' before the first build.
 A comprehensive GNU build environment is required to build the OS/2 version.
 See the docs/Readme_OS2.txt file for more information.
 
-### DOS:
-
-A build environment with DJGPP is needed as well as libraries such as
-Allegro, zlib and libpng, which all can be downloaded from the DJGPP
-website. Compilation is straight forward: use '`make`', but do a '`./configure`'
-before the first build. The build binary will need cwsdpmi.exe to be in
-the same directory as the openttd executable. cwsdpmi.exe can be found in
-the os/dos/cwsdpmi subdirectory. If you compile with stripping turned on a
-binary will be generated that does not need cwsdpmi.exe by adding the
-cswdstub.exe to the created OpenTTD binary.
-
 ### 7.1) Required/optional libraries
 
 The following libraries are used by OpenTTD for:
@@ -752,16 +740,6 @@ License 2.1, and partly under the (3-clause) BSD license. The exact licensing
 terms can be found in src/3rdparty/os2/getaddrinfo.c resp.
 src/3rdparty/os2/getnameinfo.c.
 
-The exe2coff implementation in os/dos/exe2coff is available under the
-GPL, with a number of additional terms. See os/dos/exe2coff/copying and
-os/dos/exe2coff/copying.dj for the exact licensing terms.
-
-The CWSDPMI implementation in os/dos/cwsdpmi is distributed under a
-custom binary-only license that prohibits modification. The exact
-licensing terms can be found in os/dos/cwsdpmi/cwsdpmi.txt. The sources
-for these files can be downloaded at its author site, at
-http://homer.rice.edu/~sandmann/cwsdpmi/csdpmi5s.zip.
-
 CONTRIBUTING.md is adapted from
 [Bootstrap](https://github.com/twbs/bootstrap/blob/master/CONTRIBUTING.md)
 under the [Creative Commons Attribution 3.0 Unported