when passing a fileobj into tarfile.open() without
specifying the type open() will try openers in their
dict key order from TarFile.OPEN_METH.  This can fail
if the order changes.

This patch adds a tell() and seek() on failure of
openers.  fileobjs must already support these methods
to work so this makes no new requirements.

OpenOrderTest uses a dict-alike with shuffled keys to
try and provoke the wrong orders for openers.

Let me know if it looks good and I'll check it in.