Adopt bgpctl code to the ibuf changes done in bgpd.

Adopt bgpctl code to the ibuf changes done in bgpd.

Adjust code that calls nlri_get_prefix() and friends to work with
an ibuf. show_mrt_update() is mostly converted because of this.
The output functions and the rest of the mrtparser are just
minimally touched to keep the diff reasonable.
OK tb@