Change protocol socket fd to Socket::Handle

[straight-shoota]

This fixes a couple of refactors missed in #10706. Socket uses Handle as a platform-independent handle type. The protocol sockets should do the same.
On Linux, Handle is Int32, so this change has no real effect, but it enables those methods to work on windows.

[HertzDevil]

Wouldn't this expose an internal type name to a public API?

I'm asking because I remembered a similar issue with File, but couldn't find it here.

[straight-shoota]

I don't think Handle should be supposed to be considered internal. It's an abstraction to platform-specific socket handlers. As long as the API exposes methods that return or receive such handlers, that type is exposed as well. Well technically, it's an alias and thus transparently resolves to the platform-specific type at compile time. But it serves as the generic, portable type for these handlers.

[yxhuvud]

In practice, the underlying handles will be needed to interact with external libraries, so they can't really be totally private.

[beta-ziliani]

Sounds like a reasonable abstraction. I didn't check that it's consistently applied.