Don't mark folds closed while finding top fold to close.
Switch fold open values to be YES/NO instead of true/false.
Fix comment in closeFoldAtLocation: to reflect what it's actually doing.

We find the *bottommost* open fold with the same start as
the fold at the location, not the topmost one.
Fix openFoldAtLocation:* to correctly find the topmost closed fold.

Before we were finding the bottommost closed fold instead.