Freeciv

Add initial, mostly dummy, version of tiledef.[ch] module.

Tiledefs are essentially list of properties that a tile must have. They will make it possible to require that the same tile has multiple extras etc. in requirement ranges larger than a single tile.

Currently, without tiledefs, if we have e.g.:
"Extra", "Trading Post", "City"
"Extra", "Road", "City"
there's nothing forcing the Trading Post and Road to be on the same tile, as long as there's at least one of each in the city area.

With tiledef, we can have something like
[tiledef_xxx]
name = "Example"
extras = "Trading Post", "Road"

and then requirement like

"Tiledef", "Example", "City"