Re-org/simplify the api used to add nhgs via zapi messages
from daemons, using a temporary nhe so we can convey all nhe
attributes.

Signed-off-by: Mark Stapp <[email protected]>

Add a flags field to the zapi nexthop-group struct; encode
and decode the field.

Signed-off-by: Mark Stapp <[email protected]>

Add a 'recursive' flag field and corresponding config option
for nexthop-groups.

Signed-off-by: Mark Stapp <[email protected]>

Update sharpd's internal api so that we carry more config info
when processing nhg config callbacks. This is used to allow
access to, e.g., the flags field.

Signed-off-by: Mark Stapp <[email protected]>

Add support for recursive resolution of nexthop-groups: iterate
over a group's nexthops, use the zebra validity-checking logic,
and update as necessary. This is driven by daemon-owned nhgs,
allowing the use of recursive nexthops in those nhgs.

Signed-off-by: Mark Stapp <[email protected]>

Add some words about the recursive nhg config option; clean
up duplicate doc tags.

Signed-off-by: Mark Stapp <[email protected]>