New concept sector terrain

New concept sector terrain

A sector type's terrain (struct dchrstr member d_terrain) is the
sector type of its underlying terrain.  Sector types occuring in
d_terrain are terrain types, and must have their own type in
d_terrain.  Players can change sector types only to those with the
same terrain.

The builtin configuration defines terrain types sea, mountain,
wasteland, wilderness and plains.  It gives bridge span and tower
terrain sea, and everything else terrain wilderness.  Hence, the stock
game remains unchanged.

Deities can use terrain to create sector types that can be developed
only in limited ways.