Tagstat fixes for recent changes.
Change title on overpass turbo image link.
Add support for showing geo distribution of tags.

This adds all the necessary code but it is still disabled.
Add selection database which will replace *.lst files.

The *.lst files generated by the update script are problematic because they
don't work when special characters like "line feeds" are in tags. This commit
adds creation of a database called "selection.db" that contains all this data,
but it is not used yet.

Adds dependency on m4 program.