math/scaf.c, etc.: Only need 3*NPIECE words of scratch space.

math/scaf.c, etc.: Only need 3*NPIECE words of scratch space.

The extra one would have been necessary because we're calculating the
high half of a product of two (NPIECE + 1)-piece numbers, but in fact we
only need the low NPIECE pieces of the result.

Fix the commentary and the callers.