c/r: drop in-flight connections during CRIU dump

c/r: drop in-flight connections during CRIU dump

Shortly after CRIU 2.3 has been released a patch has been added to skip
in-flight TCP connections. In-flight connections are not completely
established connections (SYN, SYN-ACK). Skipping in-flight TCP
connections means that the client has to re-initiate the connection
establishment.

This patch stores the CRIU version detected during version check, so
that during dump/checkpoint options can be dynamically enabled depending
on the available CRIU version.

v2:
   * use the newly introduced criu version interface
   * add an option to disable skipping in-flight connections

Signed-off-by: Adrian Reber <areber@redhat.com>