luit: Cygwin compatibility

[Mark Kettenis]

> From: Adam Jackson <ajax at nwnk.net>
> Date: Tue, 06 Oct 2009 11:21:13 -0400
> 
> On Mon, 2009-10-05 at 19:08 -0500, Yaakov (Cygwin/X) wrote:
> 
> > Cygwin has O_NOCTTY but no TIOCSCTTY, so the tty is never made=20
> > controlling.  This raises two questions:
> >=20
> > 1) What is the point of O_NOCTTY if TIOCSCTTY is called right afterwards?
> 
> Hah!  Can't think of one.

But I can.  The TIOCSCTTY ioctl will fail if the process already has a
controlling tty (even if the current controlling tty is the same as
the tty you want to make controlling).  So you need to open the tty
with O_NOCTTY to make sure this doesn't happen.