lib,zebra: tighten up the nexthop_copy/nexthop_dup APIs

lib,zebra: tighten up the nexthop_copy/nexthop_dup APIs

Make the nexthop_copy/nexthop_dup APIs more consistent by
adding a secondary, non-recursive, version of them. Before,
it was inconsistent whether the APIs were expected to copy
recursive info or not. Make it clear now that the default is
recursive info is copied unless the _no_recurse() version is
called. These APIs are not heavily used so it is fine to
change them for now.

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