Fix Socket_setnonblocking() for Windows (At least if build with MSVC 2010)

Partially revert commit a16cff63 for file
src/Socket.c

- Do not include errno.h for Windows build
- Use errno as a local variable in Socket_setnonblocking() for Windows

Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>