Cygwin: ctty: Add comments for the special values: -1 and -2.

Cygwin: ctty: Add comments for the special values: -1 and -2.

_pinfo::ctty has two special values other than the device id of
the allocated ctty:
-1: CTTY is not initialized yet. Can be associated with the TTY
    which is associated with the session leader.
-2: CTTY has been released by setsid(). Can be associate only with
    new TTY which is not associated with any other session as CTTY,
    but cannot be associate with the TTYs already associated with
    other sessions.
This patch adds the comments in some source files.

Suggested-by: Corinna Vinschen <corinna@vinschen.de>
Signedoff-by: Takashi Yano <takashi.yano@nifty.ne.jp>