FindSpawnPos: Let mapgens decide what spawn altitude is suitable

To avoid spawn search failing in new specialised mapgens
Increase spawn search range to 4000 nodes
Add getSpawnLevelAtPoint() functions to EmergeManager, class Mapgen
and all mapgens
Remove getGroundLevelAtPoint() functions from all mapgens except mgv6
(possibly to be re-added later in the correct form to return actual
ground level)
Make mgvalleys flag names consistent with other mapgens
Remove now unused 'vertical spawn range' setting

src/mapgen.h

@@ -181,6 +181,13 @@ public:
 	virtual void makeChunk(BlockMakeData *data) {}
 	virtual int getGroundLevelAtPoint(v2s16 p) { return 0; }
 
+	// getSpawnLevelAtPoint() is a function within each mapgen that returns a
+	// suitable y co-ordinate for player spawn ('suitable' usually meaning
+	// within 16 nodes of water_level). If a suitable spawn level cannot be
+	// found at the specified (X, Z) 'MAX_MAP_GENERATION_LIMIT' is returned to
+	// signify this and to cause Server::findSpawnPos() to try another (X, Z).
+	virtual int getSpawnLevelAtPoint(v2s16 p) { return 0; }
+
 private:
 	DISABLE_CLASS_COPY(Mapgen);
 };