to write it. fwrite(3) is a noop in that case anyway, and the error
detection logic here was screwing up on the 0 return from fwrite(3)
(that return value is mandated by SUSv2 for size or count of 0).
While here, improve the error messgages given to the user.
if there are any mistakes, please report (i only speak English/Japanese).
currently, only singlebyte locales are usable.
TODO: enable multibyte handler in libc - there still are few design
discussions, but we are almost ready.
