Separate tun queue fd from interface accessors

[cbranch]

This change supports applications that want to receive or pass a tun device's file descriptor to another (possibly less privileged) application and have it usable by tokio-tun on the other side.

The alternative is to open a multiqueue tun device and temporarily open additional queues, which has a disruptive impact on existing flows received on the tun device.

This change resolves #17.