io: replace local_isatty() with a proper function __isatty_nostatus()

io: replace local_isatty() with a proper function __isatty_nostatus()

Replace local_isatty() inlined in libio with a proper function
__isatty_nostatus(). This allows simpler system-specific
implementations that don't need to touch errno at all.

Note: I left the prototype in include/unistd.h (the internal header
file.) It didn't much make sense to me to put it in a different header
(not-cancel.h), but perhaps someone can elucidate the need.

Add such an implementation for Linux, with a generic fallback.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>