New buffer API; the first installment of the conversion/replacement

New buffer API; the first installment of the conversion/replacement
of OpenSSH's internals to make them usable as a standalone library.

This includes a set of wrappers to make it compatible with the
existing buffer API so replacement can occur incrementally.

With and ok markus@

Thanks also to Ben Hawkes, David Tomaschik, Ivan Fratric, Matthew
Dempsky and Ron Bowes for a detailed review.