FIX: don't remove public right of way river tiles without a diversion

James,
the promised bug fix for PROW rivers; there are some subtleties with unnavigable rivers and water tiles in addition to the weight checking issue.

I've also changed the checking code to ignore neighbouring ways that are not themselves PROW (or water): otherwise, we might end up setting PROW status on private roads that connect to a PROW road which is being replaced.

Philip