Add dumping of unnamed unix sockets.

* Added functionality for dumping unnamed unix sockets.
  When we call CRIU with dump option, for unnamed socket we
  should pass it inode into --ext-unix-sk. Details about this problem
  described in http://criu.org/External_UNIX_socket#What_to_do_with_socketpair.28.29-s.3F

.
  Usage example:
    criu dump -D images -o dump.log -v4 --ext-unix-sk=4529709 -t 13506

* fix typo error in log output

Signed-off-by: Artem Kuzmitskiy <artem.kuzmitskiy@lge.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>