zebra: Add functionality to parse RTM_NEWNEXTHOP and RTM_DELNEXTHOP messages

zebra: Add functionality to parse RTM_NEWNEXTHOP and RTM_DELNEXTHOP messages

Add the functionality to parse new nexthop group messages
from the kernel and insert them into the appropriate hash
tables. Parsing is done at startup between interface and
interface address lookup. Add functionality to parse
changes to nexthops we already have. Add functionality
to parse delete nexthop messages from the kernel and
remove them from our table.

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