The nexthop_group_write_nexthop_simple function outputs the
interface name, because we've stored the ifindex. The problem
is that there are ephermeal interfaces in linux that can be
destroyed/recreated. Allow us to keep that data and do something
a bit smarter to allow show run's and other show commands to continue
to work when the interface is deleted.
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>