Use getaddrinfo() in kprop and kpropd, and recognize IPv6 addresses

Use getaddrinfo() in kprop and kpropd, and recognize IPv6 addresses
when setting up krb5_address structures.  kpropd still only binds to
one socket to avoid the need for a select() loop, so we turn off
IPV6_V6ONLY on that socket to ensure that IPv4 connections will still
be accepted.

Based on a patch from Michael Stapelberg <michael@stapelberg.de>.

ticket: 6686

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24134 dc483132-0cff-0310-8789-dd5450dbe970