Enable DHCPv6 in DHCP Server Add-on

from what I understand, the DHCP Server Add-on is just a container that runs dhcpd in DHCP mode.

It would be nice to have an option to enable v4 or v6 mode (-4 or -6 flag at startup), since I saw from the docs that the server supports both DHCPv6 and DHCP.

The documentation also states that the server either operates as a DHCPv6 server or a DHCP server, but not both at the same time, so maybe it would be easier to have two add-ons available, one for IPv4 and one for IPv6.

I found a good, yet a bit old fork of the original add-on with the necessary modifications made to it to make it run in DHCPv6 mode, so the developers could just implement the changes to the official add-on (or a new IPv6 version of it) without too much hustle.