Add check for water tiles in shockwave/knockback

Fix water tiles acting like walls (previously only checked movecost). Monsters knocked into water drown if they can't swim or fly. Monsters knocked out of water flop and die if they are aquatic. If the player is knocked into water, call plswim(). No handling for NPCs yet since nothing else in game seems to handle NPCs in water. I suppose I could drown them too?