RoutingTable: sort routes by administrative distance, then by metric

so the metrics are compared only if the routes have the same adm. distance.
RoutingTable: set administrative distance of directly connected interface routes
AnsaRoutingTable: remove unneeded methods

prepareForAddRoute() is not needed, because the route is always added
to the routing table. It is not used, if there is a better one.
AnsaRoutingTable: get rid of the 'showMRoute' field

The info() method of AnsaIPv4MulticastRoute is adjusted to generate the
Cisco-like format. Therefore the 'multicastRoutes' watch is the same as
'showMRoute' was before.
RoutingTable: add a factory method for creating new routes

so subclasses can change the type of routes created
for directly connected interfaces.