Added prefer_ipv4 bool parameter to resolve_name().

Added prefer_ipv4 bool parameter to resolve_name().
W2K3 DC's can have IPv6 addresses but won't serve
krb5/ldap or cldap on those addresses. Make sure when
we're asking for DC's we prefer IPv4.
If you have an IPv6-only network this prioritizing code
will be a no-op. And if you have a mixed network then you
need to prioritize IPv4 due to W2K3 DC's.
Jeremy.