lib,zebra: Use a flag to track down status for connected addrs

lib,zebra: Use a flag to track down status for connected addrs

Track 'down' state of connected addresses with a new flag. We
may have multiple addresses on an interface that share a prefix;
in those cases, we need to determine when the first address
is valid, to install a connected route, and similarly detect
when the last address goes 'down', to remove the connected
route.

Signed-off-by: Mark Stapp <mjs@voltanet.io>