[krbdev.mit.edu #6039] more cleanup needed on routing-socket code

[Ken Raeburn via RT]

Revision 20552 reduces the buffer space needed.

Still to do:

Don't monitor IPv6 routing changes if we have IPV6_PKTINFO support.

Don't monitor routing changes at all if we have IPV6_PKTINFO and IP_PKTINFO support.

Only close down sockets on addresses we no longer have; only set up new sockets on addresses 
we didn't have before.

Log a warning if we don't have struct rt_msghdr nor IP_PKTINFO.