zebra: implement protocol NHG Add/Del

zebra: implement protocol NHG Add/Del

Implement the underlying zebra functionality to Add/Del an
internal zebra and kernel NHG.

These NHGs are managed by the upperlevel protocols that send them
down via zapi messaging.

They are not put into the overall zebra NHG hash table and only
put into to the ID table. Therefore, different protos cannot
and will not share NHGs.

The proto is also set appropriately when sent to the kernel.

Expand the separation of Zebra hashed/shared/created NHGs and
proto created and mangaged NHGs.

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