Pass struct rib_entry to rde_generate_updates() instead of struct rib.

Pass struct rib_entry to rde_generate_updates() instead of struct rib.
With this the newbest and oldbest arguments can go since the infromation
is part of the rib_entry. Especially the prefix in the rib_entry is
always valid so simplify some code in various functions below to use
this information.
OK tb@