Force TechRacer maps when image map is selected. Fixes Google Code issue #882
If you start a TechRacer map with an image map selected, the resulting map will likely to be completely unplayable.
So this commit disables this.
173069dd7b17
Make amGirder invisible in construction mode since it allows placing anywhere in fortress. Print "insufficient power" if placement was blocked due to that, to avoid confusing noobs.