lib,zebra: add api to enforce nexthop sort order when copying

lib,zebra: add api to enforce nexthop sort order when copying

Add an api that creates a copy of a list of nexthops and
enforces the canonical sort ordering; consolidate some nhg
code to avoid copy-and-paste. The zebra dplane uses
that api when a plugin sets up a list of nexthops, ensuring
that the plugin's list is ordered when it's processed in
zebra.

Signed-off-by: Mark Stapp <mjs@voltanet.io>