More auth updates. Preparing for DTLS support.

There will be a lot more changes before I'm done. I'm
considering renaming the `smcp_auth` stuff to something
like `smcp_sec` or `smcp_security` or whatnot, due to it being
more than just authentication.

DTLS support is the big thing that I'm working toward at the
moment, rather than the funky digest-ish authentication
mechanism that I was originally intending.

I'm still not quite sure how it will all work out yet,
but I'm getting closer.