Merge #tracks-tunnels and #footbikecycle-tunnels into #tunnels

The layers #tracks-tunnels and #footbikecycle-tunnels are merged into #tunnels.
This is a preliminary step to merging the definitions for normal roads, tunnels, and bridges.

Consequences for rendering:
Rendering order of footways etc., paths and tracks is now determined by z_order instead of explicitly defining footways etc., paths and tracks to be rendered lowest.
Therefore, minor order changes between service roads, footways etc, paths, tracks in tunnels might occur.
Add missing key 'tracktype' to sql
Merge branch 'roadsv5b' of https://github.com/math1985/openstreetmap-carto into math1985-roadsv5b