seize: don't wory if a cgroup contains some extra tasks (v3)

A freezer cgroup can contain tasks which will be not dumped,
criu unfreezes the group, so we need to freeze all extra
task with ptrace like we do for target tasks.

Currently we attache and send an interrupt signals to these tasks,
but we don't call waitpid() for them, so then waitpid(-1, ...)
returns these tasks where we don't expect to see them.

v2: execute freezer_detach() only if opts.freeze_cgroup is set
    calculate extra tasks in a freezer cgroup correctly
v3: s/frozen_processes/processes_to_wait/

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>