File descriptor needs to be closed on VPN shutdown

[bemasc]

In #283, the responsibility for closing the VPN file descriptor was assigned exclusively to outline-go-tun2socks, once the VPN is started. This is fine, but it means we need an explicit close action when we are done with the VPN. Otherwise, the file descriptor will remain open, and the VPN will remain active, until the Go garbage collector gets around to finalizing the File object. In practice this appears to be a very long wait (minutes).