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

[Jason A. Donenfeld]

On Mon, Oct 19, 2015 at 8:24 PM, Trevor Perrin <trevp at trevp.net> wrote:
> uglier and only helps this case, not others..

In practice it's really just that one:

33: Noise_IK_25519_ChaChaPoly_BLAKE2s
32: Noise_IK_25519_ChaChaPoly_SHA512
29: Noise_IK_25519_AESGCM_BLAKE2s
28: Noise_IK_25519_AESGCM_SHA512
27: Noise_IK_448_AESGCM_BLAKE2s
26: Noise_IK_448_AESGCM_SHA512