prepare 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, make it build again

socket_net.cpp needed some small fixes to build again,
because of imcompatibilities between Winsock and the
real POSIX sockets API.