restore: Learn to work with shared struct file-s

Some process can share one struct file-s, we may find them by "object IDs".
A file descriptor is opened in one process and send to other via unix socket.

The procedure of restoring files contains four stages.
* Collect data about all file's descriptors
  On this stage we find process which will restore a file descriptor and
  create a list of processes, who should get this descriptor.

* Create datagrams unix sockets
  If a file descriptor should be received, a unix socket is created
  instead of it.

* Open file descriptors
  A process with the least pid opens a file and sends this file
  descriptors to all one who wait it.

* Receive file descriptors.

When we were thinking up this algoritm, we wanted to minimize a number
of context switches. A number of context switches is proportional of a
number of processes.

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