The IO Context has to be alive longer than the other parts
of the state. Otherwise a deadlock can happen between shutting down
the IO context (thread join while the state lock is held) and stuff
happening on the IO context (which might take the state lock).

It's called automatically for us already.

So put it into an unsafe block.

Otherwise both would be closing the same socket, which a) breaks the
second user of the socket if any and b) could on the second close cause
a completely unrelated socket to be closed.

Windows part of the code is untested.

After a `cargo clean` the shared library is not yet present if the test is started with `cargo test`.

And switch to the new built-in spawning support of CurrentThread

And share more code between unicast/multicast socket creation