Add a confirmation box before a vehicle is sold to vehicle export

(Linked to https://mikescnr.com/forum/index.php/topic,9800.msg46583.html)

Personal vehicles are able to be sold to the vehicle export place by simply being drove into the marker, without being clear that the personal vehicle would disappear from your /v list permanently. I think that there should be a confirmation box explaining the consequences and whether you would like to continue, for example:

"You will receive $exportvalue for your $vehicle. If this is a personal vehicle, you will not be able to request it upon exporting. Would you like to export this vehicle?"