zebra/ldpd: introduce ZEBRA_ROUTE_ALL wildcard route type

zebra/ldpd: introduce ZEBRA_ROUTE_ALL wildcard route type

The ZEBRA_ROUTE_ALL route type can be used by a client to request
all routes from zebra. The main motivation for introducing this is
to allow ldpd to get routes from all OSPF instances, not only from
the default one. Without ZEBRA_ROUTE_ALL, ldpd would need to send a
ZEBRA_REDISTRIBUTE_ADD message for each possible OSPF instance (65k),
which doesn't scale very well.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>