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