In kpropd, when getting a wildcard address to listen on, try IPv6

In kpropd, when getting a wildcard address to listen on, try IPv6
explicitly (with AI_ADDRCONFIG specified where available, to avoid
IPv6 on hosts with no IPv6 interface) and then fall back to IPv4.
Only set IPV6_V6ONLY on the listener socket if the resulting address
is IPv6.

Note: we have mostly confirmed that OpenBSD does not have dual-stack
support, meaning that it would be better to open separate IPv4 and
IPv6 listener sockets, as we do in krb5kdc and kadmind.
Unfortunately, the complicated iprop retry-and-backoff logic makes
this less than straightforward.

ticket: 6686

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24156 dc483132-0cff-0310-8789-dd5450dbe970