admin: Update the nginx example configuration file to cope with IPv6

The existing configuration silently fails for any server with AAAA
records in DNS for the domain, when the client has IPv6 enabled. This
took me several weeks to debug, on and off.

This configuration should work for all servers IPv6 enabled, even if
they lack IPv6 connectivity. People who have done strange manual things
(e.g., setting disable_ipv6=1 or removing / blacklisting ipv6.ko) may
have problems, though.