design choices for a loadable module interface

[Sam Hartman]

Like Nico, I have a reasonably strong preference for
function-per-interface rather than vtables.  The main reason is that
it's easier to code the plugin and we want to optimize for simplicity of
creating plugins.

I've considered the arguments about how to handle version changes, and
tend to agree with Nico.

Vtables are nice if you want to consider static loading.

I will note this is a departure from the approach we took for the
preauth plugins and that was a concious decision, although I don't
remember whether it was our decision or simply something we accepted
when we accepted the code from RH.  The authdata interface was
explicitly modeled on the preauth interface.

--Sam