wasi: sockets, use Conn.Control on Unix, too; align to Windows

[evacchi]

The current implementation of WASI sockets differs between Unix and Windows. On the Unix version, we save underlying FD to the struct, while on Windows we keep the Listener/Connection as a field, and invoke Conn.Control(func(fd)).

The reason for the difference on Windows, is that Windows won't return a valid file handle, because the Dup implementation is a no-op. However, further somewhat related testing (see #1891) highlighted that the Unix strategy does not really work: the FD that is being saved, is not really the underlying FD of the Listener/Connection struct, but rather the FD of a duplicated File.

Because the duplicate is not being saved, but only the FD, it looks like eventually the new File instance is collected, invalidating the underlying FD. This is only visible in long-running tests, where we probably give a chance to the runtime to run a GC cycle.

The implementation on main is incorrect anyway, so in this PR I am aligning the implementation strategy to the Windows version: we keep the Conn/Listener around in our own File struct, and only access the FD through Conn.Control. This in turn allows to share a lot more of the original code, further cleaning up the implementation.

[gaukas]

Thank you, it works perfectly.