lib, zebra: Add ZAPI_NHG_ADD|DELETE

lib, zebra: Add ZAPI_NHG_ADD|DELETE

Add the ability to send a NHG from an upper level protocol down to
zebra.  ZAPI_NHG_ADD encompasses both the addition and replace
semantics ( If the id passed down does not exist yet, it's Add,
else it's a replace ).

Effectively zebra will take this nhg passed down save the nhg
in the id hash for nhg's and then create the appropriate nhg's
and finally install them into the linux kernel.  Notification
will be the ZAPI_NHG_NOTIFY_OWNER zapi message for normal
success/failure messaging to the installing protocol.

This work is being done to allow us to work with EVPN MH
which needs the ability to modify NHG's that BGP will own
and operate on.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>