ZeroMQ's IPv6 support isn't enabled by default, resulting in
"No such device" errors when attempting to listen on an IPv6
address. This change adds a ipv6 option to the ZeroMQ module
and enables it by default. Further, adds a test configuring
everything to listen on IPv6 ::1 as well, and one test to provoke
the original error. This also regularizes some error messages.
The addr_to_uri() calls weren't actually needed, but they apparently do
not hurt and the result is easier on the eyes, so use them :-)
