stdio-h: Work around fwrite bug in msvcrt.

stdio-h: Work around fwrite bug in msvcrt.

Reported by 松延 英樹 <maznobu@gmail.com> in
<https://github.com/mlocati/gettext-iconv-windows/issues/52>.

* lib/stdio.in.h (gl_consolesafe_fwrite): New declaration.
(fwrite): When msvcrt is in use, use gl_consolesafe_fwrite.
* lib/stdio-consolesafe.c: New file.
* lib/stdio-write.c (fwrite): When msvcrt is in use, use
gl_consolesafe_fwrite.
* modules/stdio.h (Files): Add lib/stdio-consolesafe.c.
(Depends-on): Add stdckdint-h.
(configure.ac): Define condition GL_COND_OBJ_STDIO_CONSOLESAFE.
(Makefile.am): Arrange to compile stdio-consolesafe.c.
* doc/posix-functions/fwrite.texi: Document the workaround.