Fix warnings from backwards-loops

[ZedThree]

Changes backward-loops over lists* from this pattern:

int i;
size_t length_of_list = /* ... */;
for(i=length_of_list-1;i>=0;i--) {
...

to this:

size_t length_of_list = /* ... */;
for(size_t i = length_of_list; i-->0;) {
...

This form enables using an unsigned type for the loop index, which usually fixes a couple of sign conversion warnings.

---

* or at least those that currently trigger sign-conversion warnings in the for statement or in the loop body

[WardF]

Interesting, and good catch, thanks! Looking at this now; I was out for the past week so there is a bit to catch up on.