Network manager can create routes at metric aka priority above 20000.
These can stick around if they are not the best choice, or they may
disappear quickly.
Do not consider one of these routes as a valid default route for
has_network purposes.
The existing event based method of watching for has_network has a flaw.
The incoming route_change events from probert do not distinguish routes
on the same interface but a different metric, so if 2 routes on one
interface appear, we only get one event. Then if one of those routes is
removed, we will inappropriately remove this route from the
default_routes list.
Aside from the code watching the event stream, the set of default routes
is an elaborate boolean value.
Simplify the code by passing around a boolean, and when we get a
route_change event, use that to go looking again at the list of default
routes.
LP: #2004659
Dan Bungert