Enable more networking specs on Windows

[HertzDevil]

With the event loop merged, a whole bunch of networking specs can be enabled now. Some are disabled or changed:

• HTTP::Server reuses the TCP port (SO_REUSEPORT): This constant is not defined on Windows. The closest thing is probably SO_REUSEADDR | SO_EXCLUSIVEADDRUSE. This is not implemented here yet.
• HTTP::WebSocket negotiates over HTTP correctly: The specs try to connect a TCP socket to 0.0.0.0, and ConnectEx does not support this; these specs now use 127.0.0.1. But see also https://superuser.com/a/1307057.
• OAuth2::Client get_access_token_using_*: Those specs ultimately rely on non-blocking IO::FileDescriptors returned from IO.pipe. It seems file reads and writes on Windows are still blocking.
• The others I don't fully understand why they fail yet.

The definition of LibC::Linger is copied from WinSock2.h.

std/socket/unix_server_spec.cr and std/socket/unix_socket_spec.cr are enabled because these files start with a skip_file now. Eventually we should do this to the rest of the failing spec files instead of having a separate win32_std_spec.cr, but that would be too much for this PR.

[sdogruyol]

Thank you @HertzDevil 🙏