Map generation limit: Rewrite

The previous implementation applied the setting to blockpos_over_limit(),
objectpos_over_limit() and in createSector(), causing many bugs near the
world edge.

First revert the previous implementation.
Rename blockpos_over_limit() to blockpos_over_max_limit() for clarity.
Add a new function to mapblock.h called blockpos_over_mapgen_limit() that
checks against the map_generation_limit setting, and call this only from
the code that decides where mapgen stops.
Use MAX_MAP_GENERATION_LIMIT in objectpos_over_limit() to reduce the
chance of bugs, there is no need to use map_generation_limit here.

src/emerge.cpp

@@ -606,7 +606,7 @@ void *EmergeThread::run()
 			continue;
 		}
 
-		if (blockpos_over_limit(pos))
+		if (blockpos_over_max_limit(pos))
 			continue;
 
 		bool allow_gen = bedata.flags & BLOCK_EMERGE_ALLOW_GEN;