Fix minetest.find_nodes_in_area() coord clamping

Fix minetest.find_nodes_in_area() coord clamping

Previously, minetest.find_nodes_in_area() was affected by a signed
integer overflow, which was fixed by clamping the area in which the
function works to MAX_MAP_GENERATION_LIMIT & -MAX_MAP_GENERATION_LIMIT.

The problem with that approach is that nodes can exist and even be generated
past MAX_MAP_GENERATION_LIMIT in vanilla Minetest – which, despite the name,
does not specify exactly where the map generator stops working. Therefore,
the bug fix created an unknown amount of other bugs near the map border.

At minimum, those bugs affect the outer mapblocks, where nodes past
MAX_MAP_GENERATION_LIMIT can be generated. Ironically, the first thing
broken by the faulty bug fix was a test case belonging to a workaround that
prevents minetest.find_nodes_in_area() being called with coordinates out of
s16 bounds in the Mineclonia game, thus avoiding signed integer overflow.

Using INT16_MIN+1 & INT16_MAX-1 makes minetest.find_nodes_in_area()
work for all coordinates that worked before the faulty fix, avoiding a
situation where Minetest can place a node in the map, but then not find
it afterwards using minetest_find_nodes_in_area().