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.

src/map.h

@@ -379,6 +379,7 @@ class ServerMap : public Map
 	/*
 		Blocks are generated by using these and makeBlock().
 	*/
+	bool blockpos_over_mapgen_limit(v3s16 p);
 	bool initBlockMake(v3s16 blockpos, BlockMakeData *data);
 	void finishBlockMake(BlockMakeData *data,
 		std::map<v3s16, MapBlock*> *changed_blocks);