a given address family and a peer only supports the family localhost does
not support. For example: configure a kernel without IPV6, and then
add a line in ntp.conf server <ipv6addr>. We report that the server is
unreachable and we keep going because there might be more servers around?
XXX: What if it is the last server? Should we detect this? It is not nice
to just bail on this error, because a server might lose its ipv4 address
and only advertise ipv6.