stdio: don't require ignore_value around fwrite

stdio: don't require ignore_value around fwrite

This patch works around libc bug 11959
<http://sources.redhat.com/bugzilla/show_bug.cgi?id=11959>.
Without this patch, applications must often write
ignore_value (fwrite (...)) even though the ignore_value is
not helpful here.  It's common to write many objects, using
fwrite/printf/etc., and then use ferror to detect output error.

I considered making this patch optional, but decided against it,
because libc is obviously being inconsistent here: there is no
reason libc should insist that user code must inspect fwrite
return's value without also insisting that it inspect printf's,
putchar's, etc.  If user code wants to have a strict style where
all these functions' values are checked (so that ferror need not
be checked), we could add support for that style in a new gnulib
module, but in the meantime it's better to be consistent and to
support common usage.

* lib/stdio.in.h (rpl_fwrite): Define this wrapper around fwrite,
to work around libc bug 11959, if __USE_FORTIFY_LEVEL indicates
that we are compiling in checking mode, and if not C++, and
if not already wrapping fwrite for some other reason.
(fwrite): #define to rpl_fwrite if the latter is defined.