Commit

65c09a96 "Set acceleration only once in falling node"

has made the acceleration being set only once.
But this has introduced a regression.

Fix #3884.

Fixes: http://i.imgur.com/arAWw1i.png (middle-right)

On the lua side, notably minetest.env:<function>(<args>) should now
be replaced by minetest.<function>(<args>).
The old way is and will stay supported for a long time.

Also:
Update and clean up lua_api.txt (by celeron55)
Move EnvRef to lua and remove add_rat and add_firefly (by kahrl)
Add separate src/util/CMakeLists.txt, other minor fixes (by kahrl)

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.