Prevent players from choosing start points on unpassable terrain for commanders

[beherith]

As they currently can, and will get stuck if they do :( Even a warning message is sufficient.

[beherith]

reproduce this

[Bluestone]

Newbie placer was not involved there - the purple guy had valid TS data from SLDB and was not classified as a newb (despite having lobby rank 0). He placed his *own* startpoint on the cliff xD

(When NewbiePlacer acts you won't see a startpoint at all; the player will appear ready with no startpoint and will have one picked for him only after the 3,2,1 countdown. It already has something in it to prevent it putting coms on impassable terain.)