Tweak comment
Don't declare set_blocking in tappet.h

It's an internal function (like read_hexkey) and we don't need it
elsewhere any more.
A first draft of what encryption support will look like

Still a noop, and API still incomplete (we need to pass k and have a
separate function for nonce initialisation/handling). But at least it
reads well.
Add a note to the README about what the crypto is and isn't