This patch replaces ctty constants with more descriptive macros
(CTTY_UNINITIALIZED and CTTY_RELEASED) rather than -1 and -2 as
well as checking sign with CTTY_IS_VALID().
Fixes: 3b7df69aaa (Cygwin: ctty: Add comments for the special values: -1 and -2.)
Suggested-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>