Connect creates firestack-administered tunnel. `fd` is the TUN device. The tunnel acquires an additional reference to it, which is released by Disconnect(), so the caller must close `fd` and Disconnect() to close the TUN device. `linkmtu` is the MTU of the underlying link (actual network). If <= 0,
(fd, linkmtu, tunmtu int, ifaddrs, fakedns string, dtr DefaultDNS, bdg Bridge)
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