SimpleIPv6: New app for proof-of-concept IPv6 packet forwarding.

Intended to interact with a Linux neighbour in the simplest way imaginable:

1. Respond to Neighbour Solicitation with a Neighbour Advertisment.
2. Route IPv6 packets back to their sender (simple demo).
3. Check hop_limit and send ICMPv6 on excess (for traceroute).
4. Respond to ICMPv6 pings.

Only partly implemented and even less debugged ...

Includes a selftest based on a pcap file that's also checked in:

    # cd snabbswitch/src
    # ./snabbswitch -t apps.ipv6.ipv6
ipv6 selftest: Generate selftest-output.pcap.

Contents look pretty sane after a couple of small fixes.