Relax shutdown() receiver to &self

[dilluti0n]

Unblocking WinDivertRecv() from another thread is in fact the whole purpose of WinDivertShutdown(), but &mut self made that impossible on a shared (Arc) handle. It only reads self.handle and doesn't invalidate it, so &self is sound.

Downstream usage: https://github.com/dilluti0n/dpibreak/blob/e14866ca04ad2909c456411c301cb8a9434c2473/src/platform/windows.rs#L42-L48

fn shutdown_all() {
    for h in RECV_HANDLES.lock().expect("mutex poisoned").iter() {
        if let Err(e) = h.shutdown(WinDivertShutdownMode::Both) {
            crate::warn!("windivert: shutdown: {e}");
        }
    }
}

[Rubensei]

This is a breaking change, and in 0.7 the signature has already been changed to fn shutdown(&self) -> ShutdownHandle, being the return type both Send and Sync