lib,zebra,sharpd: add code for backup proto-NHs but disabled

lib,zebra,sharpd: add code for backup proto-NHs but disabled

Add the zapi code for encoding/decoding of backup nexthops for when
we are ready for it, but disable it for now so that we revert
to the old way with them.

When zebra gets a proto-NHG with a backup in it, we early fail and
tell the upper level proto. In this case sharpd. Sharpd then reverts
to the old way of installation with the route.

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