default-route-manager: fix handling for absent default routes for VPN

When a VPN has no default route, it is wrong to enforce the absence
of a default route on that device. Instead, if there is no default
route, NMDefaultRouteManager should just forget about the route.

This is especially important, because for VPN types like openswan
there is no distinct tunnel interface. Instead, it shares the ifindex
with the parent-device.
Note that devices usually only enforce their default-route for a short
time and afterwards switch to non-synced. If that happens and there
is a VPN that enforces the absense of the default route on that device,
we end up deleting the default route.

(cherry picked from commit 16d0850c37)

src/nm-default-route-manager.c

@@ -789,7 +789,13 @@ _ipx_update_default_route (const VTableIP *vtable, NMDefaultRouteManager *self,
 					}
 				}
 			}
-			synced = TRUE;
+			if (nm_vpn_connection_get_ip_ifindex (vpn) > 0)
+				synced = TRUE;
+			else {
+				/* a VPN connection without tunnel device cannot have a non-synced, missing default route.
+				 * Either it has a default route (which is synced), or it has no entry. */
+				synced = default_route && !never_default;
+			}
 		}
 	}