If IPv4 or IPv6 pktinfo is unavailable, we set up a UDP listener socket 
on each interface address so that we can send reply packets from the same 
address they were sent to.  This is done using krb5int_foreach_localaddr.

krb5int_foreach_localaddr is also used by krb5_os_localaddr, which is 
used for ticket address restrictions.  As such, it excludes loopback 
interfaces, per RFC 4120 section 7.1.  But there is no reason to exclude 
loopback interfaces when setting up KDC and kadmind listener sockets.

This restriction would break most test setups under Linux, except that 
Linux has IPv4 pktinfo support.