zlistx comparators should return -1, 0, or 1

[chu11]

Per comment in #6370

The zlistx comments say

// Set a user-defined comparator for zlistx_find and zlistx_sort; the method
// must return -1, 0, or 1 depending on whether item1 is less than, equal to,
// or greater than, item2.

We actually don't do this in a nice chunk of flux, we return < 0, 0, or > 0. Examples

static int by_rank (const void *item1, const void *item2)
{
    const struct rnode *x = item1;
    const struct rnode *y = item2;
    return (x->rank - y->rank);
}

static int plugin_byname (const void *item1, const void *item2)
{
    const char *name1 = jobtap_plugin_name ((flux_plugin_t *) item1);
    const char *name2 = item2;
    if (!name1 || !name2)
        return -1;
    return strcmp (name1, name2);
}

My mini audit founds cases in rlist, taskmap, and job-manager.

This doesn't seem to affect zlistx at the moment, but should probably clean this up to adhere to requested API.