tty: split pty_open_ptmx_index()

We are going to create slave pty-s to bind-mount them,
but at this moment we don't have file_desc-s and actually
can't open /dev/ptmx by path.

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