[Tagging] sidewalk tag when mapped as a separate way

[Richard Mann]

Put the sidewalk tag on the road, and put some indicator on the
footway (I use adjacent=yes) that it's also covered by tagging on the
adjacent way.

The worst that happens is some router gets two parallel links in their
network, or that some super-clever algorithm identifies two parallel
sidewalks but doesn't have the wit to work out that they're identical.