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
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.