Fix and simplify check_trade()

Fix and simplify check_trade()

Use unit_drop_cargo() to drop a sold land unit's cargo.

Use unit_give_away() to transfer ownership.  This fixes the following
bugs and misfeatures:

* Sold nuke wasn't taken off its plane.  Could not happen before
  commit 2e40a4bb, v4.3.3.

* Nuke on a plane wasn't sold along with the plane.  Broken in commit
  2e40a4bb, v4.3.3.

* Planes and land units on sold ships got their mobility zeroed.

* Planes on sold ships didn't get their wing reset.