libbnetapi: Add BNetworkRoute to replace use of route_entry.

The BNetworkRoute class manages a route_entry and the sockaddr's
associated with it. It replaces the direct use of route_entry in the
BNetworkInterface API.

Using route_entry is fragile and inconvenient as it only holds pointers
to the sockaddr's. When getting a list of routes from the kernel, each
route_entry is set up so that its pointers point into the single flat
buffer that is passed around. Creating a copy of the route_entry and
then deleting the flat buffer makes the pointers in the copy stale.
Returning these route entries therefore always lead to a use-after-free
when they were eventually used.

BNetworkRoute also takes over the code and functionallity of getting
routes from RouteSupport. The corresponding method in BNetworkRoster is
replaced by a static method in BNetworkRoute.

Also distinguish between the default route and gateway of an interface.
GetDefaultRoute() now gets the default BNetworkRoute for the interface
while GetDefaultGateway() gets the associated gateway address within
that default route. Adjust network preferences panel to this change.

Note that we currently only seem to have per interface default routes
and not an actual global default route. This was already the case before
these changes and I did not further investigate what this means.

libbnetapi: Add BNetworkRoute to replace use of route_entry.

The BNetworkRoute class manages a route_entry and the sockaddr's
associated with it. It replaces the direct use of route_entry in the
BNetworkInterface API.

Using route_entry is fragile and inconvenient as it only holds pointers
to the sockaddr's. When getting a list of routes from the kernel, each
route_entry is set up so that its pointers point into the single flat
buffer that is passed around. Creating a copy of the route_entry and
then deleting the flat buffer makes the pointers in the copy stale.
Returning these route entries therefore always lead to a use-after-free
when they were eventually used.

BNetworkRoute also takes over the code and functionallity of getting
routes from RouteSupport. The corresponding method in BNetworkRoster is
replaced by a static method in BNetworkRoute.

Also distinguish between the default route and gateway of an interface.
GetDefaultRoute() now gets the default BNetworkRoute for the interface
while GetDefaultGateway() gets the associated gateway address within
that default route. Adjust network preferences panel to this change.

Note that we currently only seem to have per interface default routes
and not an actual global default route. This was already the case before
these changes and I did not further investigate what this means.