Don't print subnet prefix lengths and weights for one-host subnets.

Don't print subnet prefix lengths and weights for one-host subnets.

This commit suppresses subnet prefix length output (/xx) for subnets
that only contain one address (/32 for IPv4, /128 for IPv6). It also
suppresses weight information if the subnet is using the default
weight. This improves readability of net2str() output in the majority
of cases.