PATCH: Avoid excessive flushing on lower-layer handle

[Benjamin Kosnik]

Mark, I'd like this in on the branch as well. This is the output side of
Jason's recent patch for 4150.

> I'm in the middle of testing this: I'll let you know linux results as
> I finish.

Loren, great job. You're cleared for check in with this patch.

Would it be possible for you to add this scott synder test into
27_io/narrow_stream_objects.cc as a disabled, interactive test? It's
silly to have to track this down all the time. Just put in the comment
about expected output with redirection and without it to explain how to
manually run the testcase.

Cleaning up the io testsuite is high on my priority list post-release.

thanks,
benjamin