this simplifies the code, make the configuration more intuitive, at
the cost of making the labels on the layers automatically generated
and therefore less customizable
* fix will_render calls to pass proper relative filenames
* fix urls to kml etc files to not assume wiki's top is at /
* avoid building the javascript to display the map in two different
ways between the cgi and on-page maps
* refactor duplicate code
Add an underlay for the osm plugin.
Update links to right path to icon. Note that the osm plugin has a
pervasive bug in how it links to icons; it assumes the site is at /.
I did not attempt to fix that; it should be using urlto() to make a correct
relative link.