[Tile-serving] [osm2pgsql] Multi line example (#273)

[Sarah Hoffmann]

In output_multi::process_relation() `outtags` is redeclared [here](https://github.com/openstreetmap/osm2pgsql/blob/master/output-multi.cpp#L366), that can't be right. The tags are probably meant to be filtered again.

Also, we need to activate more compiler warnings.

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