toollib: select the whole structure if at least one of its items is selected

toollib: select the whole structure if at least one of its items is selected

This applies to:
  - process_each_lv_in_vg - the VG is selected only if at least one of its LVs is selected
  - process_each_segment_in_lv - the LV is selected only if at least one of its LV segments is selected
  - process_each_pv_in_vg - the VG is selected only if at least one of its PVs is selected
  - process_each_segment_in_pv - the PV is selected only if at least one of its PV segments is selected

So this patch causes the selection result to be properly propagated up to callers.