Net: use the new findloop for loop detection.

I've removed the old algorithm (the one described as 'footpath dsf' in
the findloop.c appendix comment, though I hadn't thought of that name
at the time), and replaced it with calls to the new API.

This should have no functional effect: there weren't any known bugs in
the previous loop-finder that affected currently supported play modes.
But this generality improvement means that non-orientable playing
surfaces could be supported in the future, which would have confused
the old algorithm. And Net, being the only puzzle as yet that's played
on a torus, is perhaps the one most likely to want to generalise
further at some point.