restore: --restore-detached implies CLONE_PARENT

We need to use CLONE_PARENT to prevent processes from immediately dying due to
pdeath_sig when they are restored in detached mode.

[ xemul: One more place which requires check for restore-detach
         is in sigactions preparation ]

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>