Better support for IPv6

Pi-hole currently lacks some support for IPv6;

  1. Trying to install Pi-hole on an IPv6-only host fails due to web services on install.pi-hole.net not being IPv6-enabled. This can be worked around by using a proxy.
  2. Even if web service on install.pi-hole.net is enabled, there are probably other aspects that needs proxy services enabled in IPv6-only hosts (to reach IPv4-only sites). Sync of blacklists comes to mind (externally hosted sources).
  3. Not possible to add custom upstream DNS during install with IPv6 address (currently only supported via the web GUI post-install).
  4. Partially related to above; if you add a placeholder IPv4 upstream DNS during install on an IPv6-only host, the installer fails after the "Select Protocols" dialogue (due to it trying to reach the IPv4 upstream DNS, which obviously would fail).

There could be more, without having checked all aspects of the application yet.

Add support for DHCPv6, DHCPv6 reservations, possibly support for DHCPv6-PD.