`download-osm -make-dc` to use a `.poly` file for BBOX

[zstadler]

The BBOX created by download-osm -make-dc using pbf-based statistics is larger that the bounding box of the polygon used to create the extract. In a relatively small area the bbox was 18-20 times larger.

Geofabrik - 18x

Source	West	South	East	North	Area
Polygon	34.079	29.377	35.915	33.351	7.296
PBF statistics	25.149	22.000	36.900	33.484	134.943

OpenStreetMap.fr - 20x

Source	West	South	East	North	Area
Polygon	34.050	29.435	35.915	33.355	7.311
PBF statistics	25.149	22.000	36.900	34.686	149.073

It would be nice if make-dc will use the bounding box of the extraction polygon by default, and use the statistics-based bounding box only if it cannot be found. Currently poligon files are available from Geofabrik.de, OpenStreetMap.fr, and bbbike.org