[FIX] purchase_requisition: broken procurement wkf

If a purchase requisition is created instead of a purchase order for a procurement order, the method make_po should not return the id of this requisition, as methods calling the make_po method expects ids of purchase orders, not purchase requisition ids.

Besides, on purchase requisition done or cancel, we set the procurement order accordingly, without the use of the workflow. We consider this as crappy, the proper fix should be the use of a dedicated workflow for purchase requisition, but I cannot be done in stable branches. This aims trunk/master.