ripd: retrofit the 'neighbor' command to the new northbound model

ripd: retrofit the 'neighbor' command to the new northbound model

Make rip_neighbor_add() and rip_neighbor_delete() return northbound
error codes since their return values are used as the return value of
some northbound callbacks.

These functions shouldn't fail in normal conditions because the northbound
layer guarantees it will never call the 'create' or 'delete' callback
more than once for the same object. Hence any failure in those functions
would indicate an internal inconsistency that needs to be investigated
(by returning NB_ERR the northbound will log a detailed error message
indicating the xpath of the object, the event and the callback where
the error happened).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>