tty: use a pair of dev and rdev to identify a terminal

We can't use only a terminal device, because we can not distinguish
two pty-s from different mounts in this case.

$ mount -t devpts -o newinstance xxx pts1
$ mount -t devpts -o newinstance xxx pts2
$ stat pts1/0
Device: 27h/39d	Inode: 3           Links: 1     Device type: 88,0
$ stat pts2/0
Device: 28h/40d	Inode: 3           Links: 1     Device type: 88,0

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>