rewrote posix_net.cpp based on win_net.cpp

And suddenly hosting a server on linux works at least locally
(with client and server on the same machine).
Even though there are still strange bugs (massive lags in one
direction, doesn't work in LAN), at least it works at all now.
Print net checksums as unsigned int

because it actually *is* unsigned and the negative values look strange
Fix sending clients from lobby to game when lobby and game are on same server

In the current case (only "direct" lobby backend, i.e. connect to a
server directly), lobby and game are always on the same server anyway..

It used to send the IP of the first network interface.. that kinda works
on Windows and FreeBSD in LANs (i.e. not over the internet or even
behind a NAT), but not at all on Linux, because the first device seems
to be the loopback device there (at least on my machine)..
Now it sends net_ip (so it should even work behind NAT) or, if net_ip is
set to "localhost" (the default), 0.0.0.0 is sent, which the client
interprets as "just use the IP of the lobby you're already connected to"
Don't use net_port as port to connect to

the connect commands supports adding the port with ":"
like "connect 10.1.2.3:27016" - if no port is set, it defaults to 27015

net_port is still used as the port to listen on and to send from.
Add CVar r_displayGLSLCompilerMessages to suppress AMD driver warnings

The AMD drivers output a lot of useless warnings when compiling the shaders.
Those are pretty annoying, especially as idRenderProgManager::LoadGLSLShader
prints out the whole shader with them..
So I added this CVar to suppress them (when it's set to 0)
Add Winsock support to posix_net.cpp (to remove code duplication)

win_net.cpp and posix_net.cpp were almost identical, i.e. caused a lot
of duplicated code.
To get rid of that, unify both files - by adding Winsock support to
posix_net.cpp and, in the next step, moving posix_net.cpp to sys/common/
and removing win_net.cpp
Move sys/posix/posix_net.cpp to sys/common/socket_net.cpp, remove win_net.cpp

and adjust the CMakeLists.txt accordingly
preparre unification of more sys/win32/ and sys/posix/ stuff

because the code is almost the same anyway, so
* add win32 support to sys/posix/posix_signin.cpp
* add posix support to sys/win32/win_savegame.cpp
Move stuff from sys/posix/ and sys/win32/ to sys/common/

All these files were almost identical, so there is no good reason to
have them twice..

and change CMakeLists.txt accordingly

(Not that this commit won't compile because some #includes are still
broken - will be fixed in the next one)
Adapt #includes for moved files