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/emerge.h

@@ -164,6 +164,8 @@ private:
 	bool popBlockEmergeData(v3s16 pos, BlockEmergeData *bedata);
 
 	friend class EmergeThread;
+
+	DISABLE_CLASS_COPY(EmergeManager);
 };
 
 #endif