zebra: revise pw reachability logic

zebra: revise pw reachability logic

Modify the pseudowire reachability logic so that it returns
success if there is at least one installed labelled nexthop for
the route resolving the pw destination. We also check for
valid backup nexthops if necessary, in case there's been a
switchover event.
Only OpenBSD requires that _all_ nexthops be labelled, so we
have a more strict version of the logic also.

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