[Tree] Error out and do not add friend if kEntriesReshuffled is set

The TTree::kEntriesReshuffled bit signals that a TTree is the output
of the processing of another TTree, and its entries are reshuffled
w.r.t. to the original TTree. A TTree with this bit set should not
add friends nor should be added as a friend, because it is very likely
that the original event-by-event correspondence has been broken.

Users can manually unset the bit to force the original behavior.