[Tile-serving] [openstreetmap/osm2pgsql] Rename file format option and simplify default (auto) case (#1196)

[Sarah Hoffmann]

`auto` is an explicit name that can be used for the option. This is also stated in [the help string](https://github.com/openstreetmap/osm2pgsql/blob/master/src/options.cpp#L198).

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