Monitor default routes from platform, and resync the default routes
on changes.

For one, this fixes the following use-case: have an assumed device em1
with two routes of metric 20 and 21. Activate em2, which will get effective
metric 22.
When externally removing route em1/20, em2 would resync the effective metric to
20. This is correct and already worked before. However, when deleting em1/21,
nothing happened. With this change, em2 would resync to metric 21 to fill the gap.

However this commit has much bigger effects: whenever the user externally adds
a default route to an interface for which NM manages an default route, NM will
delete it.
Also, when deleting the default route (managed by NM), NM would readd
it. Effectivly, the user can no longer mess with the default route on
interfaces for which it manages the default route.

If the connection is configured never-default, the user still can add
default routes and NM will not touch them.

Obviously, this has no effect for assumed devices either and the user
can externally add and remove default routes as he wishes.

https://bugzilla.gnome.org/show_bug.cgi?id=735512



Signed-off-by: Thomas Haller <thaller@redhat.com>