[PATCH] Fix libstdc++/12048

[Loren James Rittle]

> This should fix libstdc++/12048. There are still some corner cases
> where strange behaviour may be observed, but these all involve calling
> unget when the last I/O operation wasn't a read operation on cin. The
> new XFAIL tests check for two of these.
>
> Tested on i686-pc-linux-gnu.

Petur, thank you for the attention to details while fixing this (9 new
test cases beyond my simple one).  Please commit (I note that Petur is
not listed in write-after-commit, can someone help as I'm not near a
committing computer until mid-next-week).  I understand the
reservations on additional outstanding corner cases (-5) but you have
clearly recovered the 99%.

Regards,
Loren