* When IPv6 is enabled, check that system services are accessible over IPv6 too and that reverse DNS is setup correctly for the IPv6 address.
* When IPv6 is enabled, check that system services are accessible over IPv6 too, that the box's hostname resolves over IPv6, and that reverse DNS is setup correctly for IPv6.
* Explanatory text for setting up secondary nameserver is added/fixed.
* DNS checks now have a timeout in case a DNS server is not responding, so the checks don't stall indefinitely.
* Better messages if external DNS is used and, weirdly, custom secondary nameservers are set.