This collapses, per road type, definitions that are the same for normal
roads, tunnels and/or bridges.
In addition, it gives some instances clearer names.
This makes it more easy to maintain the code, and to detect inconsistent
Changes in rendering:
* Render minor service roads on bridges and tunnels smaller and on higher
zoomlevels only, just like minor service roads that are not on a
bridge or tunnel. This solves #269 on Github and 3883 on trac.
* Restore bridges of bridleway, footway, cycleway, path, lightrail on