[noise] Reworking PSK usage

[Jason A. Donenfeld]

On Wed, May 3, 2017 at 7:55 PM, Trevor Perrin <trevp at trevp.net> wrote:
> I'll try to update the rev32 branch today or tomorrow.  I'd like more
> feedback, but hopefully we can publish revision 32 next week, since
> rev32 contains a bunch of good text cleanups that I'd like to get out
> soon.

Great. FYI, the symbolic model of the new PSK model proves just fine
(for IKpsk2, at least).