stack-tracker: Make lower_below / raise_above internal as well

These are unused elsewhere.
stack-tracker: Fix an off-by-one error in restack_managed

When restacking the last window alone, we would trigger this off-by-one
error. This would throw us off the end of the array, causing lower_below
warnings for nonsensical values.

Since the last window already is lowered below everything else, we
shouldn't need to lower it.