[Tile-serving] [osm2pgsql] Set polygon/linestring differently for some key values differently than the key in default style.lua (#346)

[math1985]

@mboeringa Good point about historic=citywalls.

So basically the problem is that a closed linestring tagged with historic=citywalls can mean two things. Either it is a circular wall, or a wall of which the outline is tagged. Unfortunately, both of them are used commonly in practice. The question is now what we should assume as default.

Unfortunately the presence of a building tag is not sufficient to tell the two apart. For example [Templin](http://www.openstreetmap.org/#map=19/53.12111/13.49609) has a citywall mapped as area, but no building tag. And conversely, I could also imagine that an area tagged with both a building and citywall tag might signify a building with a city wall around it (like a fortress).

To repeat the question: a citywall as an open linestring will in any case be interpreted as line. What should we do with citywalls as closed linestring?
A. Treat as line, unless tagged area=yes.
B. Treat as area, unless tagged area=no.

I think personally I'd prefer option A. It seems counterintuitive to me that closing a linestring would suddenly change the semantics. Also, I don't think we currently have any tags that can refer both to a line or area that do not need an area=yes tag when used as area, except the tags in this PR.

What do others think?

For breakwater and groyne, the situation is similar, except that closed linestrings for these objects in almost all cases do refer to an area. Should we treat these objects differently?

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