Support tunnel interfaces

[tkjaer]

Support tunnel (e.g. wireguard) interfaces.

if iface.Flags&net.FlagPointToPoint != 0 {
    // Likely a tunnel/VPN interface
}

references:

• issue with tunnel interface google/gopacket#99

[tkjaer]

This doesn't work fully on OpenBSD yet.

Maybe not on NetBSD either.

[tkjaer]

OpenBSD’s WireGuard interfaces don’t expose a libpcap-compatible datalink, so every pcap_sendpacket invocation from gopacket fails with EAFNOSUPPORT; as a result, our traceroute probes can’t be transmitted over wg0 on OpenBSD until we bypass libpcap for that platform.

• Instrumenting pcap_sendpacket showed OpenBSD returning EAFNOSUPPORT even after switching the interface to its NULL/LOOP datalinks and prepending the expected pseudo headers.
• Writing packets via a tun device wasn’t possible because WireGuard on OpenBSD doesn’t expose a tun file descriptor.
• Bypassing libpcap entirely and transmitting over a raw IP socket is the only current idea for an approach that might succeed