We can MSG_PEEK those and can dump the files easily. The
problem is in restore -- we need to first pass through all
the fdinfo openings before sending the descriptors out, but
by that time some sending sockets may have already closed :(

So for now just detect SCM-s in queue and abort the dump,
otherwise we'd just have them lost and the state spoiled.

https://travis-ci.org/criupatchwork/criu/builds/177945936Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>