libstdc++/4150: catastrophic performance decrease in C++ code

[Benjamin Kosnik]

> Here's a more limited patch, which just avoids the seeking.  Rather
> than define _M_in_end to indicate the file pointer, it adds another
> member.

Your previous patch included a modification to
testsuite/27_io/filebuf_virtuals.cc, which was correct: please include
it. It doesn't change the output of make check.

This patch wins. I have verified that there are no new regressions.
Please check it in to branch and trunk immediately.

thanks,
benjamin