Main change in this patch is the introduction of FloodplainnStream::send() and
fplainn::Zudi::send(), which are basically mirrors of one another, but simply
to be called from different types of processes.

* Cleaned up fplainn::Endpoint::anchor()'s first argument by adding a union
  to replace the untyped first argument.
* fplainn::sChannelMsg now has a back-end function which is called on by the
  other two send() APIs.