Add restoring of unnamed unix sockets.

Added functionality for restoring unnamed unix sockets
using already implemented feature - inherit fd and using same command line
option.
Usage example:
criu restore -d -D images -o restore.log --pidfile restore.pid -v4 \
     -x --inherit-fd fd[3]:socket:[9677263]

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