Be more precise in nonce generation rules for the protocol description.

Eradicate some potential misunderstandings, by making entirely sure we got the right kind of
nonces in all places.
Fix a gen_serverism: noreply → next_state.
Only reply back with 'ok' once when the connection comes up.

This fixes a bug in the code where the wrong case was hit. There seems
to be a better solution based on the idea of separating the state reply-back
out from the "generic" part of the code.
Be better at correctly interpreting nonce values.

Fix a bug.
Fix nonce-sending in vouches.

We accidentally sent the wrong value over the wire, which lead to trouble.
Correct side-handling in nonces in the protocol.

We now correctly identify each side. This makes sure the protocol does the
right thing with client/server side nonces. If this is not done, the protocol is
not secure: the nonce is reused, destroying the confidentiality of the protocol!