Option to disallow walking on street when separate sidewalks present

[ansoncfit]

In some places, OSM has sidewalks mapped separately from the main carriageways:

When a sidewalk is mapped separately, add the tag sidewalk=separate to the carriageway it borders – this prevents mappers from describing the sidewalk twice. https://wiki.openstreetmap.org/wiki/Guidelines_for_pedestrian_navigation#Sidewalks_and_crossings

It's straightforward to disallow walking on such carriageways, but we should allow users to specify whether they want to build networks with the stricter walking traversal permissions, because it may affect results in unexpected ways.