nerf patrol node farming

nilsge reports:
17:56 < nilsge> there is one AI flaw I am able to constantly exploit. If you
                shoot an alien, for example in the ufo, from the door, then in
                the next round another alien will go to that exact place
17:57 < nilsge> if it can be avoided they should try to not stand on fallen
                comrades. Don't go where another one was just shot

This code marks nodes where an alien has been shot as "dangerous" and does not allocate them again (except in base defense missions)