Add multicast support to UDPSocket on Windows

[stakach]

Comments inline for where I got the documentation
Also cross referenced with the rust documentation for constant values as MS has two definitions, used ws2 defs

[stakach]

So weirdly specs pass on the previous commit.
Not sure why they didn't pass on the crystal tool format commit

[straight-shoota]

I'm rerunning them. Maybe it's just a fluke? We've experienced some random failures with network specs before.

[stakach]

rerunning them worked, probably overlapping as they hadn't completed when the next test run started

[HertzDevil]

IP_MULTICAST_LOOP behaves differently between Windows and Unix-like systems. How much of that affects UDPSocket on Windows?

[stakach]

@HertzDevil Practically I don't think it matters much, typically you are using multicast to send data somewhere else. Loopback is really only used if you as the sender want to process the data being sent so you can separate your producers and consumers.

If you have two apps multicasting on the same machine using the same port then you have to enable loopback on both of them for cross platform compatibility. Unusual edge case I guess, but probably rare you'd ever have a setup like this and not enable loopback on both processes.