Call this to redo an operation that was undone by UndoNetLastOper_bn.

After N calls of UndoNetLastOper_bn and then N calls of RedoNetOper_bn, the net will be in the same state as it was before the calls.

Returns 0 or greater if it succeeded, otherwise negative. The most common reason for failing is that all the operation that were undone have already been redone.

Pass -1 for to_when; it is only for future expansion.