Fix cargo giveaway in scrap and scuttle

Fix cargo giveaway in scrap and scuttle

When giving away cargo by scrapping or scuttling its carrier, the
cargo's cargo wasn't given away.  Happened for instance when a ship
carrying a land unit carrying a SAM got scrapped.

Also, wing, army and mission weren't cleared.

To fix, create unit_give_away() and use it in unit_drop_cargo().