Listen on IPv6 by default

By default, it's perfectly safe to bind to the IPv6 interface by default
(avoiding all the stupid "ipv6only=yes"-but-only-once stuff you usually
have to do with nginx).

Linux will listen on IPv4 as well on this interfaces, so long as
net.ipv6.bindv6only is off, which it is by default.
openspending: add upload.openspending.org config