The idea behind is pretty simple -- once we find
that there is a controlling terminal present we
do call ioctl on appropriate /dev/pts/N.

This is done in a bit unusuall manner. When we
find that there is a controling terminal present
we do create an additional FdinfoEntry for it
with object id taken from existing master peer.

The file engine stack this new FdinfoEntry on
fd_info_head head list. Thus we will have at
least two entries on this list. One for real
Fdinfo associated with master peer and one for
our new generated Fdfinfo entry, it depends on
pid which one become a file master.

Finally we do use post_open_fd hook in our
tty code which allows us to open controlling
terminal and yield proper ioctl on it.

v2:
 - restore control terminals via service fd,
   still need to speedup service fd retrieval.

v3:
 - use prepare_ctl_tty() helper to generate
   control terminal fdinfo entry

v4:
 - use post_open_fd
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>