188.8.131.52: revisit FIND/POSITION bounds checking on lists
The spec doesn't actually /require/ us to signal an error if we
are not in danger of running out of bounds as far as I can tell.
So don't traverse the entire list during forward searches if we
find what we are looking for before specified :END or the list end.
Patch by Alec Berryman.
This revisits lp#452008, and fixed lp#554385.