[CRIU] pgids/sids support in crtools

[Pavel Emelyanov]

Hi,

I've pushed support and test for the subj. The support is still not complete, not
all combinations are supported. All the problems we still have come from the
inability to attach to group/session with ID no tasks own as its PID. This can be
workarounded by fork()-ing this pid temporarily, but we'd rather think in the
direction of modifying the kernel to give us direct syscall for this (oh my...)

Next step is ptys and the screen tool and ssh sessions support by criu (TCP is on
its way, yes) :)

Thanks,
Pavel