s3: Add support for AF_NETLINK addr notifications

s3: Add support for AF_NETLINK addr notifications

Via an AF_NETLINK socket, the Linux kernel can inform us when IP addresses are
added or dropped.

This will first be used in winbind, it was triggered by clustering with ctdb.
When winbind is connected to a domain controller and ctdb decides to move away
the IP address that winbind used locally for the connection to the DC, the next
request will run into a timeout. winbind sends out its request, but the
response will never arrive: The IP is gone.

It will also be interesting for more reliable online/offline detection, but
this is something for future winbind refactoring.