criu: add an option to skip in-flight connections

Trying to migrate containers with tomcat and running ab as a test client
(ab -n 1000000 -c 20 http://url/to/test

) criu sometimes exited with an
error if in-flight connections were detected. Criu can handle sockets
listening and with established connection. In-flight connections can
happen on sockets which do not yet have a full established connection
(SYN, SYN-ACK, and the last ACK is missing).

This adds a new option to criu:

 --skip-in-flight  this option skips in-flight TCP connections.
                   if TCP connections are found which are not yet completely
                   established, criu will ignore these connections in favor
                   of erroring out.

With this option criu will skip sockets in this state and let's the client
handle the re-connection.

Signed-off-by: Adrian Reber <areber@redhat.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>