Added new chat function `in_hell [<player_name>]` and used it to find the bug. The issue was that `local function obsidian_teleport(player, pname, target)` failed to test `nether.trap_players` before modifying `players_in_nether`.
Mapgen: Find a way to get the perlin noise values inside [-1; 1] or use
another noise.
The problem is visible in the nether forest, where the mapgen code flattens
the ceiling if it is very high.
Mapgen: Generate more detail inside the simple pyramid-like "buildings",
e.g. add a small treasure chest node which contains items