key blinding: Use consistent terminology

Unhelpfully, the spec uses the variable name h and the phrase "blinding factor" for both the unclamped and clamped value. The clamped value is internal to the algorithm.

But our code used the words "parameter" and "param". Despite having reviewed this originally in !980 (merged), I spent a while this morning trying to figure out what these things were so I could call the right one(s).