Map generation limit: Make per-world

Map generation limit: Make per-world

The setting limits map generation but affects nothing else.
Add 'mapgen_limit' to global mapgen parameters.
Move 'blockpos_over_mapgen_limit()' to the only place it is called
from: map.cpp.
Allow teleportation to any part of the world even if over the set
mapgen limit.
Simplify the reading of this limit in mgvalleys.
Remove the 'map_generation_limit' setting.