On systems with struct rt_msghdr, open a routing socket and wait for

On systems with struct rt_msghdr, open a routing socket and wait for
messages; when they come in, if the types suggest a possibility of
network interface reconfiguration, shut down the KDC's networking and
bring it back up again, rescanning the interfaces in the process.

Leaving the ticket open because it should be improved:
 * It should only close down sockets on addresses we no longer have, and
   bring up sockets only on new addresses.
 * If we have IPV6_PKTINFO support, it should only listen for IPv4
   routing changes.
 * If we also have IP_PKTINFO support, it shouldn't be used at all.
 * If we build a KDC on a system with neither struct rt_msghdr nor
   IP_PKTINFO (do we have any such?), we'll need another solution.

Thanks to Nico Williams for the routing socket suggestion, and Apple
for the initial (signal-driven) reconfiguration code.

ticket: 6019
status: open

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20540 dc483132-0cff-0310-8789-dd5450dbe970