Fix memory leak.

Dense block cache was never freed. As it is a single static allocation
that is needed nearly throughout the program, it isn't a major issue.

Found thanks to valgrind.
Fix --help to give the correct --cache-strategy for chunk

also error out if an unknown --cache-strategy is given
Add a .gitignore file including all the autogenerated files
Make what goes into planet_osm_roads configurable with lua

Moved the processing of z_order and what goes into planet_osm_roads
into the tag processing and allow to make it configurable via lua.

The ways function and relation tag transform functions now return an additional
flag to specifiy if a feature should be added to the roads table