SO_SNDBUF: don't set SNDBUF for win32 versions vista or later

The Microsoft knowledge-base article
http://support.microsoft.com/kb/823764 describes how to use SNDBUF to
overcome a performance shortcoming in winsock, but it doesn't apply to
Windows Vista and later versions. If the described SNDBUF magic is
applied when running on those more recent Windows versions, it seems to
instead have the reversed effect in many cases and thus make libcurl
perform less good on those systems.

This fix thus adds a run-time version-check that does the SNDBUF magic
conditionally depending if it is deemed necessary or not.

Bug: http://curl.haxx.se/bug/view.cgi?id=1188
Reported by: Andrew Kurushin
Tested by: Christian Hägele