Added ducks. Also added river spawns from GROUP_RIVER.

Quack.

Adds ducks, because we're rather lacking in birds.

I then noticed that there were no river spawns, despite having a GROUP_RIVER, so I added some basic statements to spawn river groups.

However, that's going to be impeded by the fact that the game refuses to generate monsters in water, because it has a move cost of 0 (see #3676), but that's a separate issue entirely. They'll still generate on river banks, at least.