Add support for relays without an IPv4 address

A relay with only IPv6 address(es) doesn't work for several reasons. That's OK for quite a while -- we probably don't want a network with relays which can connect only to a (smallish) subset of the other routers.

But there are also limitations in the code today making a relay use the mandatory IPv4 address in ways it shouldn't have to. An example is a bridge requiring that an IpV4 address is configured (but not necessarily being used) in order to work.

This enhancement ticket collects the kind of bugs which are not strictly showstoppers for a mixed IPv4/IPv6 network to function but which should be fixed anyway.