[noise] Revision 13: rename "S" to "K"

[Trevor Perrin]

https://github.com/trevp/noise/blob/master/noise.md

I don't remember why I changed the naming from "K" to "S", maybe it
fit better with other things at the time, but "K" seems obviously
better to stand for (N)one, (K)nown, (X)mitted, e.g.

 N  = no static key for sender
 K  = static key for sender known to recipient
 X  = static key for sender transmitted to recipient

So I changed it back, sorry (e.g.. Noise_IS is now Noise_IK...).

Trevor