zebra: clear protodown_rc on shutdown and sweep

zebra: clear protodown_rc on shutdown and sweep

Add functionality to clear any reason code set on shutdown
of zebra when we are freeing the interface, in case a bad
client didn't tell us to clear it when the shutdown.

Also, in case of a crash or failure to do the above, clear reason
on startup if it is set.

Signed-off-by: Stephen Worley <sworley@nvidia.com>