opn-arp IPv6 improvements

[Qhilm]

I forked the opn-arp.sh repository and added the ability to specify a DHCPv6 range to allow ignoring any IPv6 outside that range. Indeed, IPv6 outside the DHCPv6 range are often dynamic and make the arp table continuously grow, even crashing opnsense in my case after a while. I keep the ULA and link local IPs though.

It would however require adding two fields in the GUI (start and stop of DHCPv6 IP range).

I also noticed there was another recent contribution here: #238.

This might generate a conflict, I'm not sure. I'm happy to merge the changes, I just don't know when the code for the webGUI is for one, and second, I can't figure out how to make a pull request on the opn-arp.sh repository.