Disallow using roadmaps underground

Fixes #7744. Prints the message, "You should read your map when you get to the surface." when the player tries to read a roadmap underground.

Technical note: `overmapbuffer::reveal(const point &center, int radius, int z) ` doesn't update the overmap the way it should if z = 0 when the player is on any level other than 0. That's a bit outside the scope of this fix, but I don't think it's WAI.