Function divmod

Compute Python divmod(x, y), returning the quotient and storing the * remainder into *r. The quotient is the floor of x/y, and that's * the real point of this. C will probably truncate instead (C99 * requires truncation; C89 left it implementation-defined). * Simplification: we *require* that y > 0 here. That's appropriate * for all the uses made of it. This simplifies the code and makes