Add DISABLE_CLASS_COPY macro (and use it)

Use this macro to disallow copying of an object using the assignment
operator or copy constructor.  This catches otherwise silent-but-deadly
mistakes such as "ServerMap map = env->getMap();" at compile time.

If so desired, it is still possible to copy a class, but it now requires
an explicit call to memcpy or std::copy.

src/mapgen.h

@@ -182,6 +182,9 @@ public:
 
 	virtual void makeChunk(BlockMakeData *data) {}
 	virtual int getGroundLevelAtPoint(v2s16 p) { return 0; }
+
+private:
+	DISABLE_CLASS_COPY(Mapgen);
 };
 
 struct MapgenFactory {