zebra: zebra GR only works with AFI's limit it

zebra: zebra GR only works with AFI's limit it

We have code that tracks both afi and safi's,
but we only ever operate on the afi's.  So lets
limit our work being done to something more sensible.

I'm leaving the safi being broadcast through the zapi
message, as that I am not sure what else should be ripped
out at this point in time.

Finally re-arrange the zread_client_capabilites function
to stop the multiple levels of function calling that really
serve no purpose.

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