tty: restore sid on master points only (v2)

If a master point has sid, it doesn't mean, that all slave points
will have sid. Look at tty03 for example:

  PID   SID TT       COMMAND
26748 26748 ?        tty03
26749 26749 pts/2     \_ tty03
26750 26750 ?             \_ tty03

The second process has not a file descriptor for the ctl tty,
but this tty is opened in tty03.

v2: If a slave point with sid has not a master point, the option
--shell-job must be set and sid isn't restored for such terminals.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>