Use newly-created accessor function for reading rt prefix in ipfw.

[melifaro]

Summary

One of the goals of the new routing KPI defined in r359823 is to entirely hide`struct rtentry` from the consumers. It will allow to improve routing subsystem internals and deliver more features much faster.

This change is one of the final changes eliminating direct access to the struct rtentry fields.

In particular, it introduces new rt_export_prefix() function, exporting rtentry destination as sockaddr and mask as prefix length.

[melifaro]

Export IPv6 addresses in "normal" form.