CURL_GLOBAL_WIN32 should only control winsock #3313

CURL_GLOBAL_WIN32 is documented to control winsock initialization but controls win32_init which sounds (and is) more encompassing, as it does sspi initialization as well. I think we should split it off into win32_winsock_init for the flag and then a general win32_init that is always called, and we can move sspi init there.

This comes from my review of #3312, since it is only necessary to call QueryPerformanceFrequency once it makes more sense for us to do it at startup (like in win32_init) rather than every single time, and save the frequency in a global structure.