Adds support for multiple interfaces

[DerPeit]

This pull request adds the ability to monitor multiple lan interfaces and add dns entries accordingly. It adjusts the main script to accept an interface as a command line argument and adjusts upstream scripts (i.e. init script and ip6neigh binary) to launch multiple services, one per interface.

If you have some time to spend, feel free to review it. I did no extensive testing, though. I could have done better, but I cannot focus on this, currently. So if you're not willing to merge it, I am perfectly fine with it. Maybe it helps someone else as a head start on this topic.