Update OpenBGPD to use new ibuf API.

Update OpenBGPD to use new ibuf API.

This replaces the old way of using a static buffer and a len to build
UPDATEs with a pure ibuf solution. The result is much cleaner and a lot
of almost duplicate code can be removed because often a version for ibufs
and one for this static buffer was implemented (e.g. for mrt or bgpctl).
With and OK tb@