add UnwindSafe impls to UdpSocket

[Hodkinson]

Fixes: #4336

Motivation

Solution

[Darksonn]

Instead of adding it to just UdpSocket, I think we should add it to PollEvented, which should make all tokio::net types unwind safe.

[Hodkinson]

Makes sense, added it there instead. I also added a unit test, but can remove/refactor this if it's too much.

[Darksonn]

I don't mind adding some tests, but I think we should include a test that verifies that the trait is implemented on the actual tokio::net types rather than on PollEvented.

[Darksonn]

Seems good.