dump: rework freeze of threads

When we try to freeze threads, some of them can exit
and a few new ones can be born. Currently we unfreeze process free
int this case, so we have the same chance to failed in the next case.

I suggest to not unfreeze frozen threads, just try to update thread list
and freeze unfrozen threads.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>