Currenly crtools supports a case when a child shared a fd table
with parent.

Here is only two interesting things.
* Service descriptors should be cloned for each process
  who shared one fd table.
* One task should restore files and other tasks should sleep in this
* time.

v2: * allocate fdt_lock from shared memory
    * don't wait a child, if it doesn't share fdtable
v3: * don't move ids on the pstree image
v4: * save ids in a separate image
    * save fdinfo per id instead of pid
v5: fix alignment of service_fd_id
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>