stdio: use __REDIRECT for fwrite, fwrite_unlocked

stdio: use __REDIRECT for fwrite, fwrite_unlocked

* lib/stdio.in.h (fwrite):
When working around bug 11959, use __REDIRECT rather than '#define
fwrite(...) ... fwrite (...) ...'.  This is a more-targeted way to
fix the -Wunused-value issue with clang, and it works with GCC too.
Problem with targeting reported by Eric Blake in
<http://lists.gnu.org/archive/html/bug-gnulib/2013-05/msg00067.html>.
(fwrite_unlocked): Treat like fwrite.  I ran into this issue while
debugging the fwrite issue.