On restore find out in which sets tasks live in and move
them there.

Optimization note -- move tasks into cgroups _before_ fork
kids to make them inherit cgroups if required. This saves
a lot of time.

Accessibility note -- when moving tasks into cgroups don't
search for existing host mounts (they may be not available)
and don't mount temporary ones (may be impossible due to
user namespaces). Instead introduce service fd with a yard
of mounts.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>