CFR - fflush() behaviour change

Hi;

Out of curiosity I took a peek at Apple's libc and generated some diffs 
against
the libc in FreeBSD's 8. Most of the changes are not relevant to FreeBSD
anymore as we have grown alternative implementations for things like 
xlocale.
Nevertheless, there are still some small things to look at.

In the hope of avoiding surprises, and because I wanted to try out 
phabricator,
I have this change for review:

https://phabric.freebsd.org/D440

While the change was inspired on Apple's patch, it was cleaned up with 
help from
Bruce Evans. I did some basic testing but it can still use some more 
review/testing.

Pedro.