In addition to linking with Winsock 2, also use winsock2.h.
This allows to turn on IPv6 support on by default under MSW, so turn it
on under Unix too for consistency.
Predefine _WINSOCK_DEPRECATED_NO_WARNINGS to avoid warnings about
deprecated (but still available and very unlikely to be ever removed)
Winsock 1 functions that we still use.
Simplify things by putting setup.h files themselves under version
control and getting rid of setup0.h ones.
The initial motivation for using separate setup0.h files was to allow
having local changes to setup.h, but with Git there is a simple way to
do it by using "git update-index --skip-worktree include/wx/msw/setup.h"
for example, so we don't really need setup0.h any more and dropping them
makes things simpler.
